... and that's what makes it so important.
Really. Think about it. About the only time I ever log in to Dropbox on the web is to grab my referral link for posts like this one (if you sign up, I get more free storage!), or to set up Dropbox on a new machine.
Outside of that, I don't have to think about Dropbox. I have a folder (with subfolders!) on each of my machines. Whatever's in the folder on one machine is in the same folder on every other machine I have (okay, okay, there's a few seconds when I boot up and it's syncing, but I've never had to wait to use something on a different machine that I can remember).
If every machine I own inexplicably breaks down at the same time, I can go out, buy a new computer, turn it on, install Dropbox ... and BAM! All of my important day-to-day offline data is back. I used to try to force myself to adhere to a schedule of burning a CD of the stuff every month or so. Now I don't have to. It's up-to-date right to the last instant I used it, all the time, period.
The only reason I'm even mentioning it is that venture capitalist Bill Gurley has an interesting piece up on his blog, justifying what seems like a pretty large valuation of the company by investors. It's not, he argues, because Dropbox solved a problem that nobody else has solved yet, and has made itself one of those "essential products" by doing so.
I don't know if he's right about the valuation, but I've tested some other sync services and none even come close to Dropbox's ease of use for me. The others require more attention, they want to impose software "management client" downloads on users, etc.
Dropbox just works, elegantly, and it stays out of your way while doing so. It is to other sync services what the Mac was to the DOS command line: All of a sudden stuff that used to be complicated and burdensome got very simple. After awhile we stopped noticing that because we got used to it and everyone else was offering a reasonable (sort of) facsimile, but that doesn't diminish the importance of the original, or eliminate its edge on the copycats.