I've been using CX.com for awhile now in addition to Dropbox. My shared folders in Dropbox are automatic and all the people who share files with me have Dropbox on their machines, so the hassle factor of changing would be extreme, especially since even with us all sharing video clips and pics and such, I'm rarely over 2GB of the 5GB available.
For backing up my work files I've been using CX.com - it syncs multiple folders automatically and is as painless to use as Dropbox in most respects. Unfortunately, the sync service is giving me some headaches right now - some of my key files (luckily also backed up in Dropbox) stopped syncing some time ago, I just discovered. I've updated to the most recent client for Windows and hope that fixes the problem, since I'm using over half the available 10GB for large DOC files plus my ebook collection and would hate to shift all that over to a more restricted service like SkyDrive.
I like CX.com a lot and REALLY hope the software update makes it sync again.