I really like Dropbox - the ubiquity and ease of use are great.

There's one annoying hangup tho, and this is what so far has kept me from paying for a great service I would otherwise immediately sign up for a paid account:

When you share a file/folder in your Dropbox with someone the space used counts against BOTH your quota AND the other person's quota.

This means that even if I am paying $10/month for a 50GB account, if I share a folder that has 2GB of data with a coworker or friend that only has a free account, that share immediately fills their entire quota.

Dropbox's only answer to this problem is that you should sign up for "Dropbox for teams." This is $795/year for 5 users with a shared quota. Not really an acceptable solution in my opinion.