It's been out for a while, I have one running Arch as a backup system. It has good throughput on the gigabit ethernet and USB 3. Not as fast as a real machine, but not too far behind either. A typical desktop can hit 80-90 MB/s running rsync over gigabit ethernet and dumping to disk, I'm able to do about 57 MB/s on the XU4 to an external 7200 RPM drive on USB 3.
The processor is very powerful as well, kicks the crap out of everything else out there.
My only grievance is the fan. It's loud and is constantly starting/stopping, it can be really annoying if it's within ear shot.