You still have some computer concepts is only to the basic installation, but there are fabulous tutorials on the net. I advice the sitrib Raspbmc if you want some thing functional from the start.
MKV Full HD 1080 perform flawlessly. You can control XBMC with remote software provided, it is really convenient (Yatse on Android is not bad at all). Possibility of making airplay from an iOS device (sound only).
Mirroring is in beta (rplay) but it's still a beta.
In short, huge opportunities for such a small device.