I'm just starting testing but I notice some unexpected slowness with rsync. I reached 20 MB/sec with rsync to nfs on ext3, and now 10 MB/sec on btrfs. File creates on NFS are unusably slow (a few per second) while on the NAS itself they seem to be okay. I will do some more experiments another time.
As for being unstable: both Suse and Oracle support btrfs now. But they use a much more recent kernel.