Got mine yesterday, just purchased the necessary cords/card today.
RaspBMC itself is a very easy install. On my main PC (Linux), it was basically run a python script to copy the install files onto the inserted SD card. (Note: they recommend at least a Class 6 card; I opted for an on-sale Class 10 ('Lexar' brand name)).
On first boot (with network cable attached and card inserted), tech news free netflix
Rpi boots and d/ls the rest of what it needs, create the filesystem on the card, and boots to XBMC (~5 minutes).
XBMC menus are a wee bit sluggish on changing screens, but not annoying by any stretch. Videos play pretty much flawlessly, with some flashing of a black screen on start (an auto-adjust, maybe?).
Video tested: SD versions MKV, MP4, AVI from a USB drive, and a 1080p MKV played via the XBMC YouTube plugin. No stuttering at all.
For what I paid vs. what it does, it's pretty impressive. I almost hate to slap it on the back of a TV and forget about it.