I recently bought an ART MPA II digital. I'm using it so I can track 6 inputs at a time, through an Omnistudio/Delta 66 interface.
At first, I thought the unit was defective, because I had a hard time sync'ing the s/pdif. It is recommended to have the ART as a master and the card as a slave. Every now and then it won't sync up and I have to toy around with the settings on the ART unit or restart my pc. But once its sync'ed it'll remain that way.
Also, if you're using the digital output; make sure to check your levels/gain on the needle meters. If you're not careful you can get some pretty ugly clipping on your tracks, even if you're not getting any red leds on the digital meter.
It is recommended to change the tubes, although I have to say that I kinda liked the original tubes. This is very subjective and obviously your ears are the judge of it all. Because of this I won't discuss on how it sounds, also I have yet to try it with different mics and instruments. (But so far, I'm liking the high voltage setting with vocals very much.)
Overall, I think it's a great option to expand your setup without busting out your wallet. The sound quality might not be up there with a 1073, but it sure does sound good to me!