I have a on loacation rig that runs on a mac mini. It has worked like a charm and I have yet to have a major issue with it. I have had it up and running since October 2006. I am thinking about upgrading to Leopard and 7.4.2 but my system is so stable and reliable I don't know that I need/want to.
I have used it for all kinds of professional audio work. From live 2 track recording to studio recording/mixing using high track and plugin counts and even a live multitrack recording/dvd shoot. For that one I was recording 18 tracks for 3+ hours and didn't have a single hiccup.
http://www.soundmotionrecording.com
http://www.myspace.com/soundmotionrecording