what kind of "compromises" will I have to make if I use a KeyWiz. Also, is it any harder/easier to work with compared to the others?
Easy questions first:
Well, first IIOOIIOO's (sp?) solution above is simpler, if that is acceptable to you. Personally, I think whoever uses the gamepad's will end up feeling at a disadvantage to the person with the "real" controls. An option (just thought of) would be to mount a four port USB hub below the panel and use 4 gamepads for the 4-player, 4-person games so everyone is on an even keel, so to speak.
Harder\easier to use: I think the KeyWiz is a little simpler and the software is a little better, but they're all basically the same as far as functionality and hook-up's. I can walk you through it. Main advantage is the KeyWiz will support 4-player, 3-button games and the I-PAC will only support 4-player 2-button games. NOTE: This covers everything in MAME except about 4 games, mainly Dungeons and Dragons series, which are four player, four button.
Compromises - Will take three full posts to get you all the details, but first there are some things to decide:
If you only want to support four player three button games - (not Street Fighter or classics) then you have less compromises, basically, you end up re-mapping the encoder away from the default codeset, re-mapping MAME away from the default codeset, and doing some funky stuff to support select, escape, and pause buttons (and note I am adding six additional inputs to each encoder without using shift keys).
If you want to support Street Fighter style games, then you end up with shared inputs to throw into the mix. So for example, you have six buttons for Player 1, but TMNT only needs 3 (2?) of them, so you can map buttons 4, 5, and 6, to be Player 4 Up, Down, and Button 3. Likewise, Player 2 Buttons 4, 5, and 6 become Player 4 Right, Left, and Player 3 button 3.
I would recommend putting Player 1 and 2 in the middle for SF games. Now for TMNT, you can either have Player 3 and 4 on the outsides, or re-map MAME and have a standard 1, 2, 3, 4, but either way, the middle players can "mess up" Player 3 Button 3 and Player 4 joysticks controls by using the buttons above them. So depending on how "honest" your friends are, will determine how you set the player positions up.
(You also could probably rig some kind of hidden rotary switch to disable buttons 4-6 (on the panel) for TMNT style games, but now we're really complicating things).
Anyway, give some thought to what you want out of the panel, and then post back and I'll walk you through it. Tomorrow for sure this time (It's late here . . . )