Hi kado897, I've been using Freefilesync for a few months (after switching from SyncToy) and have mixed feelings about it. I use it to sync two external drives, mirroring the changes I make on one to the other.
I like the easy to use interface and speed and until the previous version of the software it worked really well. But with 5.5 it started crashing and coming up with lock errors in the middle of syncing sessions which then took me a lot of time to fix manually. Fortunately since I only mirror a drive, no changes were made to the source drive else I would have lost data. Found the sourceforge forums to be annoyingly unhelpful.
After the latest update (5.6) it seems to have stabilised. Though I have been treading cautiously, syncing only up to 1GB data at a time and just now tried with 2.5GB of changes without any error. I now find it hard to trust FFS with larger jobs (>10GB) but at the moment haven't found a better alternative.