At first glance, it seems a touch expensive, as you can get Github and GMail accounts that offer far more storage for less expense. But on the other hand, Dropbox don't have the advantage of being able to assume that most of their customers won't use the full 100GB, as is the case with emails or source control. Also, looking at Amazon's S3 pricing, it isn't much cheaper to use something like Jungledisk with S3 directly, and Dropbox is much nicer.
Dropbox also retains revisions and deleted files, which on free accounts at least does not count against your quota. If this is true for paid accounts, their users can end up using more than what they think they are paying for.
