The Fireface OSX drivers are post-beta now. I have one, I love it. Can't compare versus a MOTU, but the rep of RME won me out. Latency etc., on my setup, is more a function of CPU than anything else. I'm running a 550 powerbook, and it gets to hurting.
Another factor in performance is how you plug it in vis a vis drives. I'm running a LaCie Firewire 800 card for the fireface, and a OWC firewire drive off of the stock interface. Things work much better, and lower CPU in this setting than when I ran the fireface throught the drive on a shared bus.