It's my understanding that the new HDSP cards have the mixer engine embedded in the hardware so they're supposed to boast extremely low latency times.
I was shopping for an ADAT interface a few weeks back and was considering either a MOTU 2408 or the RME HDSP-9652.
has the same implementation as the other RME PCI card, so technically it should be a hair faster than the MOTU and possibly a little more stable on the PC.
Unfortunately, I can't speak of real-world performance. Things here have changed so I'm not in the market for one of these at the moment. I wouldn't even bother chiming in except I've seen this come up a few times this week and no one has mentioned the hardware mix engine thing...