My issue with "investing" to a new service, is I always wonder whether that company will stand the test of time. I got burned by buying some DRM music from, ah, can't even remember that service but it was bundled with the Dell DJ mp3 player software and backed by someone big, I think Yahoo. The company was discontinued and the music stopped working.
Anyway, now I think the same will happen for Dropbox. Essentially, they strike me as a company founded around a commodity service, that anyone else can offer. So, dropbox will be likely to end up in a war of attrition, race to the bottom situation, and go out of business. Might take a while to lose the inertia from being a first-mover, but that's just a matter of time.
Unlike one-trick-pony companies like Dropbox, someone like Google or Microsoft or other big companies that are viable for other fundamental reasons will stick around and not be doomed if they race to the bottom for that particular service. They are able to charge the rock-bottom (even break-even) prices for the service, whereas dropbox - wouldn't they have to charge a bit of a premium to make a profit, and that charge will be disproportionate because they derive all their business from that one source/service?
If you bet your existence on a commodity service that everyone else can offer, you are doomed to lose out on the ability to charge a profit.
But maybe I'm misunderstanding what dropbox is - are they more than a dumb locker/storage/commodity that everyone else can offer?
Edit: Oh, almost forgot - I still haven't committed to an online storage service, but I'm very pleased with using Google Music to put all my music on the cloud and get the ability to play all my collection on my phone, even though my phone memory can't store it all. Same with google documents, my wife and I share lots of organizational documents, such as a list of movies we want to rent, we both add to it using our phones, and if either of us visits a Redbox, we can easily pick up the DVDs we want to rent. I think for me, Google Drive makes perfect sense, and it integrates with Android phones etc.