i would say a good 60% are MKV, 10% AVI, 10% MP4, and the rest are made of VOB MOV etc.
i have the newer WDTV Live unit.
if i play movies through an external hard drive this WDTV player can take a small portion of the hard drive and write out a playlist. if the WDTV unit can play it, it can be added to the playlist.
when i connected my WDTV to my network and found it easier to bring up a movie without putting it on a hard drive, i got used to it. the problem is that the WDTV live unit cannot make a playlist without the movie files (and all the other files) being on an external hard drive. i cannot believe it cannot be done!
so i brought back my HTPC outside and then found the joy that is Cinema Experience through XBMC! XBMC itself plays back everything but the playlist doesnt allow anything that isnt a movie to be mingled in with movies. CE allows that.
i do also alternate between the two. obviously if i do an "away" movie night, the movies are on the hdd and i dont put on any ads or other vids. at home its really a matter "if i feel like it". if i "dont feel like" setting up the big screen, i bring out the WDTV and only play networked movies one by one.