I've now tried both raspbmc and openelec..
My thoughts:
Raspbmc, while being the more fluent of the 2 is somewhat lacking in functionality. The UI is quick, but it playback need some more work! Playing a 1080p mkv with 5.1 audio makes the display turn black on stutter in and out,
As with openelec, if your connection directly to the TV, go to system, settings, audio and untick the both "reciever" options and the audio will work!
Openelec is slower, but playback is flawless and even my oem remote works a treat!
One little sad point in both, no option to mux 5.1 audio to just stereo..