I'll suggest you check out XBMC's forums and see all the "mini" pc options that people are discussing. There are various itx based systems out there that perform as well (and better) than the Mac Mini.
I use a Mac Mini for some XBMC playback. I opted for Linux install because Apple decided that OSX/EFI should not support HD audio streams such as DTS-Master. If only Dolby+ and DTS proper are okay by you, then a Mac Mini with OSX will work quite well. Windows and Linux on the Mac Mini will handle files with HD audio streams without reducing them down to core Dolby and DTS.
If I recall, Zoltac, Asus and about 3 other makes have mini style computers that work very well with XBMC and similar size to the Mac Mini.
If I had the time, I would build a nice mATX or itx system with a good quality small computer case rather than use a Mac Mini. In the meanwhile, I'm considering a nice large DAS for my Mini and move my media files to it and out of my NAS units.