I recently got the K2.1 5 fin set (thruster or quad). Haven't had a chance to try the quad setup yet (the thruster set is fantastic IMO), but I've heard really good things about it (being more pivoty then some of the other sets).
Here is a link with many pages discussing FCS quad preferences. Real surfers experimenting with different fin setups and giving you their personal experiences. (started back in Jan. '09, last post Aug. '10) Lots of good info in there, if you want to take the time to go through it. Plus there are a couple of threads discussing quads.
Also,
McKee's website has some good info as well.