If you sign up via my referral link below we both get an extra 500 MB if you sign up for a free account or 10 GB if you sign up for a 30 GB account or larger:

https://www.sugarsync.com/referral?rf=hvcrnjz57xeq

Pros:
Many features, very stable, easy on system resources

Cons:
Sometimes if you choose manually to move the files to all synchronizing computers (instead of just moving them to the shared folder of ONE computer and then letting SugarSync sync the others via the internet) Sugarsync creates duplicates even though we’re dealing with completely identical files (same checksums). Sugarsync renames the duplicates according to where they we’re found again so you’ll get to versions of the same file (but with different names) on your computers.
Sometimes you can circumvent this by pressing CTRL-SHIFT-R thereby ordering SugarSync to do a clean restart. But how it really works I don’t know….

I am using Sugarsync since about 8 months or so. I am syncing an iMac, a MacBook and a Lenovo notebook with ca. 55 GB of data.
Since then I don’t have to think anymore about keeping everything in sync, and I know that all data is being backed up online.
Also Sugarsync works invisible and does not seem to slow down the PCs at all.

PS: Since you never ever should trust only one backup solution (we might go bankrupt if our data would vanish for whatever reason), I use as second solution Apple’s Time Machine and as third final secure storage solution “Jungle Disk”….