I got a ipac4 and my only problem was wanting entirely tooooooo many buttons to go through 1 poor encoder.
I use my mame cab for a few console games (since it's already 4 player) including Poy-poy for PSX, Super Smash brothers for n64, and finally, games like Secret of Mana on SNES (Gotta love 3 player RPG action!).
Since I already knew I was going to need 6 buttons for the few SNES games I have (L, R, X,Y,B,A, plus Sel and Start), I went with a whopping 8 buttons per....
Needless to say, if you use up all 8 buttons, it doesn't give you much room for other things... but then again, that's why you have the shift button!
Other than my stupid flub (which was my own stupid fault, not Ultimarc/Andy's), the ipac4 is an awesomely cool unit!
-LJ