But let's face it, this valuation of Dropbox cannot be relied on. That $4 billion is mostly derived from fat cat VCs competing for a hot deal. It's far more important to ask: who is using Dropbox currently and is there anything in the use cases that justifies such a high valuation? So I asked the tech-savvy RWW community just that.

1. People now access the Web on multiple devices; including the traditional PC, smartphones, tablets like iPad, Netbooks and more.

2. As Web-connected devices proliferated, it became increasingly useful to use cloud computing services for file management.

Startups have attempted to tap into this huge market opportunity as far back as 2006, when ReadWriteWeb named Sharpcast (now known as SugarSync, a Dropbox competitor) as our Most Promising Company. So Dropbox isn't the only file sync service around. Some of the other products mentioned in my informal survey were SugarSync, Box.net, Ubuntu One and Wuala.

However it's Dropbox that has emerged to become the leading file sync service, since launching to the public in September 2008.

Daily Users

Most of the feedback I gathered from Twitter, Google+ and Facebook indicated that Dropbox is a product that is used not only daily, but multiple times a day. That's always a great sign for a startup.

Mickey Mellen is a heavy Dropbox user. Mellen told me via Google+ that he uses Dropbox about 100 times every day. He has over 40,000 files in his Dropbox account, totalling over 30 GB in size. "It's simply my primary drive," he said. "Every file of mine goes in there, so that every device of mine (desktop, laptop, netbook, tablet, phone) has access to it."

Tori Cushing said that she uses it daily, "all through the day." She has "just about all of the folders on my computer on Dropbox."

Dylan Bland is another daily Dropbox user. He uses it as a "default directory for a lot of important saves." He added, "we also use it at work to share common files."

Campbell Yule tweeted that he uses Dropbox "all the time, many, many times a day." His main use case is "for file sharing across locations."

ReadWriteWeb's own Jon Mitchell is a heavy Dropbox user. "Nearly all my primary workflows sync across devices using Dropbox," Jon remarked. Writing is of course Jon's occupation and he told me that he works "entirely in .txt files that save to a Dropbox directory that I can get to and work on from any device."

David Acklin said that for him, Dropbox is "always running, always backing up."

Both Personal and Business Use

The responses suggested that Dropbox is being used for both personal and business file sharing. Michael Schade uses it to store personal files, although he said that many of his business files have "moved to Google for easier collaboration."

But Dropbox is getting a lot of uptake for business collaboration, too. Ben Kepes said that he uses Dropbox multiple times a day, for "collaborating on docs across timezones and geographies."

Professional photographer Gareth Robins uses it every day for a variety of uses, including a shared folder with his accountant and another with his photographer assistant. He also uses it to deliver photos to his clients. He puts the photos in a Dropbox folder and shares the link with them.

My friend Emily Davidow told me on Facebook that she uses Dropbox "regularly for multiple shared projects."

Less Frequent Users

Others use Dropbox less regularly. I myself am in that camp. I currently use Dropbox to save files (mostly PDFs) that I want to read on my iPad. However I'm interested to expand my usage, now that I've heard how others are using it.

RWW writer (and part-time musician) John Paul Titlow tweeted that he uses Dropbox about weekly, "for household file-sharing, exchanging sound clips w/ bandmates and sending very large files occasionally."

Kim Sherrell told me on Google+ that she uses Dropbox "once in a while." She noted, "I'm reluctant to share resources when I'm editing or working on big files."

Workflow & Ease of Use

As Brian Ries rightly pointed out, how much you use Dropbox depends on how integrated it is in your workflow.

The ease of use of Dropbox was another factor that people commented on. "Dropbox just works," remarked Bill Kirby.

That's an important point, because syncing in the modern Web is too often an irksome process full of glitches. Indeed ever since I "upgraded" to Mac OS X Lion, I've noticed that home sharing (Apple's way of syncing your iTunes account across devices, among other things) has been very buggy.

Like any startup that is scaling up fast, Dropbox has its challenges. Dave Parry, an Assistant Professor at the University of Texas, blogged earlier this year about privacy concerns. But overall, the feedback I gathered from Twitter, Google+ and Facebook was overwhelmingly positive. Many respondents said that they use Dropbox multiple times a day, for both business and personal file sync and sharing.

Big Market - And So Far Dropbox is Delivering...

File sync and sharing is a potentially huge market, which we identified back in 2006 when we named Sharpcast as our Most Promising Company. Dropbox has become the leader in this space, due to its generally frictionless service and excellent usability. Indeed, its usability across multiple devices is probably what has kept it one step ahead of cloud computing giants, such as Amazon, Google and Microsoft.

Whether it's worth $4 billion is another matter. Judging on the product alone though, Dropbox has carved out a great niche and it has a good chance of becoming a very valuable company. Let us know what you think of Dropbox's value, in the comments.

Just before I started writing this, I was attempting to sync data from the online note-taking app Evernote. I had made some notes on my iPad Evernote app while in a cafe, where I didn't have Internet connectivity (I'm a premium subscriber to Evernote, so I have offline access to my data). When I attempted to sync up that content to my Evernote desktop app in my home office, it didn't immediately update. I refreshed... then again... no sync. Perplexed, I moved onto another activity and then checked again 5 minutes later. By then the changes had synced up, but the delay was disconcerting.

However last night I could not get a friend to show as 'online' in imo.im, despite having been chatting with that person moments before using Facebook on my PC. After struggling with this for 5 or so minutes, I switched to Friendly and managed to re-establish contact with my bemused friend. That may've been a bug in imo.im, but regardless it was another example of things not syncing as they should.

How Syncing Across Devices Works

Syncing usually involves using the Internet as the central 'hub' - with apps and/or devices being 'spokes' that connect to the hub. Evernote's approach is fairly typical:

All of your notes (unless stored in a local -- that is, not synchronized -- notebook) are synchronized to Evernote on the Web. Evernote's servers house a copy of all synced notes because all of the Evernote client applications, both mobile and desktop, connect to Evernote on the web to get the latest version of the notes for each user.

Evernote's service and software applications are arranged in what's called a "hub-and-spoke" configuration. This means that every single sync operation that takes place will involve Evernote on the web (the "hub"). For instance, if you initiate a sync from Evernote on your desktop computer, any new or updated content will be uploaded to Evernote on the Web so they'll be ready to be downloaded when any of your other devices initiate a subsequent sync.

Specialist Sync Services

So far we've been talking about sync as a feature of web services, but there are also specialist sync services. Dropbox is probably the most high profile in the consumer market, but another is Sharpcast, which ReadWriteWeb named as our Most Promising Company of 2006.

We chose Sharpcast back in 2006 because it was "solving a big problem (syncing data across Web, desktop and other devices) and also is an integral part of many different trends that will be popular in 2007 and beyond - mobile, rich media, a world of multiple devices, and more."

While we were right about the trend towards multiple Web devices - and that was before the iPhone was unveiled in January 2007 - things haven't quite panned out for Sharpcast. It's not because syncing became unnecessary. In fact it's a must-have feature in all apps that work across devices, particularly those with an offline component such as Evernote and Instapaper. Sharpcast had an opportunity to establish itself as the sync glue for other web apps, but it didn't manage to pull that off. Instead, the likes of Evernote and Instapaper built their own sync solutions.

Sharpcast is still plugging away, but now it competes with Dropbox and others as primarily an online storage service. Sync is one of the main features, but it's no longer enough on its own.

Sync Remains a Problem

Evernote is one of my favorite apps (here's an interview I conducted last year, which outlines how the product was created) and most of the time the sync works well on it. But syncing in this day and age should be completely hidden from the user. I notice some kind of sync issue with Evernote every week or two. It still has work to do.

More generally, I am constantly coming across issues where things don't sync up properly between devices. Skype has probably been the worst offender for me over the years - for example, adding someone as a Skype contact on one computer and then not seeing that person display when you log into another computer. To this day, I continue to come across that 'bug.'

What have been your experiences of sync? More importantly, have you come across any promising new sync solutions?

The service, which offers a free 45 day trial, costs $49.99/year for 10GB of storage with plans running up to $249.99/year. The company is running a special 50% off the first year promotion, but 200 ReadWriteWeb readers can have the promotional price extended to two years by clicking this link.

SugarSync provides both a PC and Mac client, as well as mobile clients for the Blackberry and Windows Mobile and a browser-based application.

How Does It Perform?

SugarSync is the product of nearly four years of work by the crew at Sharpcast, and the result is quite nice. It's an easy to use system that can free your data from a single, physical location. It has some nice features, such as the ability to share photos with friends, or the ability to lock certain versions of files online so that even if they're updated locally you still have a backup of the original. It also has some things we wish it did better, such as automatic file versioning, which is absent in this release.

In January, we said that Sharpcast reminded us of Feedburner, "in that it wants to become the leading platform for a new type of media service that only came into being in the Internet age." In Feedburner's case that service was RSS, whereas for Sharpcast that technology is file syncing.

Unlike Feedburner, though, SugarSync is entering a slightly more crowded space. They face competition from BeInSync, Windows Live FolderShare, and the currently in beta Y Combinator startup Dropbox. Even so, SugarSync comes to the table with a very nice product that is super easy to use and quite polished (it should be after 4 years!), so we still expect that our earlier prediction that it could be a future acquisition target to hold true.

Have you tried SugarSync? What about any of its competitors? Which do you like better? Let us know in the comments below.

Note: An earlier version of this article erroneously stated that SugarSync costs $49.99 per month for the 10GB package. That is incorrect. $49.99 is the yearly price at that storage level.