This thread is a little old, but figured I'd give my 2 cents anyways.
Like Stu, I'm using XBian on my Pi and have a few different computers networked to it. So far I'm not having any issues. I use network shares from my main computer, and my roommates, to watch videos in the living room. Over a network cable I have not seen any lag. I don't have a WiFi dongle to test out the wireless, but my wireless sucks anyways. There are only a couple of places where I can say I have experienced "lag". When trying to access the menu during playback it takes 3-4 seconds before it displays, but that's trivial in my opinion. Also, after closing the menu while the video was still playing in the background, I have noticed a slight increase in the playback speed as if it were trying to catch up, but that might just have been because of the amount of time I had spent in the menu while troubleshooting a sound issue. In case you are curious, I had it plugged into the wrong input and then not plugged in at all. Spent an hour trying to figure that out.
Besides that, everything runs very smooth. Menu transitions are fluid, even during playback. Audio and Video sync up. Setup is pretty much nill. I had to adjust my screen alignment, but I am also using an older TV. The only problem I am faced with now is DVD playback and figuring out how to get the samba "shortcuts" to the proper sections, if that is even possible. All-in-all, I think this was a wonderful investment that beats my prior investment in a PS3, that I don't really use anymore, hands down.
Besides, it has so much potential! I can't wait to start pushing this puppy to the max!