A lot of us on hacker news have more than 100GB of data to back up. I use Dropbox all the time but have no need for more than the free account. Dropbox is only for syncing. If I need to back something up it goes to CrashPlan, where I store hundreds of gigs of data, with no ceiling, for the same price as 100GB / year on Dropbox.
I don't trust Dropbox enough to store more than a few gigs of data in it and I have 10GB for free. All it takes is a username and password to get into there, so I only store stuff in there that's convenient to sync between home and work and that isn't particularly sensitive.
So far, I've only had one experience with dropbox, and it was mediocre. Someone('s dad) was trying to share a ~1.5 gig file with up to 50ish people (mostly artists and musicians). The free account used up all its bandwidth and the person had to apologize to everyone and ask that they please wait while they figured out what to do (I assume they were using a free version and needed to upgrade, but that's just a guess. The Dropbox faq doesn't say anything about bandwidth).
So I uploaded it to amazon s3 and configured direct access to the bucket via the web (a very easy procedure though somewhat technical). It cost me less than $5 to get that file to the remaining people, with little worry about service interruption.
See that really isn't the main use case for dropbox, dropbox is more like seemless backup and sync.
You have a dropbox folder on your computer you work on stuff in, everything gets uploaded and revisioned while you are saving files and when you change computers everything is there.
You also get pretty good folder sharing with others and a web interface.
Personally though I've stopped paying for dropbox and might consider going back if they implement client side encryption for some users, but til then its waiting for a fast enough good enough replacement or for dropbox to implement it.
Most of the large stuff I have, TV Shows, Movies, Music, Programs are pretty easily replaceable and given I don't do any major amount of graphics or produce much video the main thing that I'm using dropbox for is code which easily fits within the nearly 8 gig limit after all the bonus space on the free account.
I sometimes wonder if they have been to generous, it's one service I would definitely pay for if it had a lower free cap.
I don't think of Dropbox as anything more than a sync service, and I don't have enough data stored on there to worry about going to a pro plan.
For backup I use backblaze, and I have about 100GB backed up across 2 computers (so $10/month) which I'm ok with. That's mainly pictures, music, documents, etc. I don't backup anything that's easily replaceable.
