I started with BTSync, but it would eventually get out of sync and stop working. I have no idea why.
Now, on a similar note, Dropbox also gets out of sync for some reason on it, but at least I can fix it quickly by removing the directory from sync and then adding it again. Also, BTSync would do it weekly, whereas Dropbox usually takes longer to get hosed.
I don't know, something about the Whatbox makes it impossible to keep fully synced for more than a month or two at a time. With BTSync it was a lot harder for me to get it going again, so I eventually switched. Also, unless I'm doing some 80GB porn rip, the upload to Dropbox is typically really fast, and it has the added safety net of it being up on Dropbox no matter what happens to your other computers. I'll admit, maybe BTSync would be as easy now that I know more about it, but I don't think so. I've got it working now, for the most part, so close enough.
EDIT: Anyway, that's not the reason I actually pay for Dropbox, it is just an added benefit. I have a ton of other shit I like to keep synced in there, including several hundred gigs of my music that I share between home and work.