I tried Skifta with Rock Player Lite and too many of my xvid's just wouldn't play. Then I found UPnPlay last night on the market. That with Rock player played everything in my collection, even Mkv's although they just can't stream (extreme chop) through the limited n chip in the Xoom (72Mbps max). I've tried everything I could get my hands on and UPnPlay + Rock Player Lite is the best performing streaming solution for the Xoom so far. Even seeking or jumping to a certain point in the by clicking on the timeline in Rock Player works flawlessly and immediately, an important feature for me.
I was using VLC S&C pro previously, but it's like a broken beta app compared to using UPnPlay + Rock Player. Highly recommended!