If so, how does it work? In particular stability (both program and OS) and issues, sonic and otherwise. I am thinking of getting one, but only if it works well.

I used to (828mk2).  They work great.  Sound wise they are above average and the software is very nice.  I had no problems with stability at the end.  

I'm currently using a Motu Ultralite Mk III Hybrid - it is rock solid. Never have had issues with the drivers.
 
Beyond that, the feature set: mixer, monitor FX, etc. are tops - you'll never have an issue with latencies during recording. Tremendous value in it price class.

I'm actually looking at an HD Express since I feed my system in to a AV receiver because I'm a total fanboy of Audyssey room EQ. Hence I need an HDMI output. As the sig indicates, I use a pro-sumer card right now (a Creative Labs X-Fi with an HDMI out bolted on) which seems to work fine but I'm considering a real pro card. Not much available in the pro market with HDMI out though.

I have a motu 828 mkIII hybrid..connected via firewire.. flawless

828mkII working flawlessly with Sonar for the past 5-6 years here.

Having lots of troubles currently with the UltraLite-mk3.  Blue Screen after Blue Screen.  I've got a ticket in with MOTU and I do hope they can come through for me !!

I'll make my long story short: the UltraLite mk3 Hybrid is now working EXCELLENTLY with my ASUS G75V Notebook running WIN7 Home Premium 64 with 12G Ram.
 
MOTU put up a Driver update just yesterday.  I REMOVED the older driver and installed the new and everything is working perfectly.
 
Thanks to CAMPBELL at Motu for sticking with me.  :-)
 
 

Have the UltraLite mk3 (non-hybrid, fw only) and works flawlessly with very low latency

You guys like ASIO or WDM for Sonar with your MOTU gear? My HDX-SDI shows up today, barring any UPS stupidity :)