I have been running Fedora 21 for a while now on my main laptop. I have also been testing some of the TCs and RCs as time permits.With release candidate 1, I think lots of things are fairly ironed out. I think Fedora 21 is going to be a fantastic release!
The remain "major" bugs (officially, blocker bugs) are listed here -
These could use some testing with the RC images.
Go Fedora! :D