The Knight's of Trinity play it on week 6 (we have a six week campaigns and start the next block after) and generally if we're finished on week 6 we take in silly card games which include Munchkin, Gimme the Brain and Flux.
Munchkin is a great game for when you and your freinds are just plain bored. It lends itself to many jokes and is always a good time. The wizard I feel is the best class. But if you want to kill and hour or two Munchkins is definately worth buying.
A gnome is one of the best races in the game also. Adding a monster to your level and bonuses is pretty great. i use to have all the sets till they where ruined by the rain. Stupid rain.
I've never played the game myself, though a friend recently purchased the Munchkin d20 books, and was reading me some choice excerpts....it sounds hillarious. I'll be keeping my eyes open for any used copies I might come across.
As for the card game? Nah, the people I run with are pretty much all M:tG, all the time.