Quick update - I tried Openelec (31st July build), xbian 0.2, and tomaz's deb version last night. The only one I had no issues at all with was tomaz's deb which played back everything I tested without any problems (AVI with dolby, MP4 with AAC, 720p x264 with 5.1 dts, 1080p x264 with 5.1 dts). The menus were also very smooth.
Xbian was a close second but stuttered on the 1080p. I did have to lower the overclocking because it was crashing every minute or so requiring a reboot at the default settings.
For some reason, openelec stuttered every few seconds for everything except the MP4. Obviously this build is working for a lot of people, but for me last night it was terrible.
So for now I'm sticking with tomaz's deb. I'll test RC4 of raspbmc when its out though
Edit: just noticed that xbian 0.3 is out, so I'll give that a try tonight