I currently have 2 panels, and i'm getting ready to start on a third.
Panel 1;
I switchable joystick, 4 buttons, one trackball, one spinner. Its configuration effectively plays a large number of games comfortably, without being overly confusing.
Red button- used for 1 button games.
Grn & Blue button - used for 2 button games.
Yel, Grn & Blue buttons - used for 3 button games.
If you really wanna get nuts and play a 4 button game, I suppose you could, but this panel isn't really configured for that.
Panel 2. I only use this for side by side 2 player games (fighters, etc. ) It's impossible to remember what button does what, and guests get very confused if this is used on anything except fighting games.
I'm not really interested in modular panels for the following reasons;
1. Some designs take too long to swap
2. Some designs don't attach well enough
3. Don't like the big cut lines that are visible.
4. I'm not a complete purist... don't really care if things are completely accurate.
I just wanna have room for expansion, I want things to be easy for my friends to setup & change, and I want to have good aeshetics on my cabinet.
Someone asked about connectors... I use 37 pin connectors... I solder the wire to them myself. I think that 21 gauge wire is the best for arcade stuff (except for power & ground on 18) Using phone lines, ethernet, and printer cables are fine, but I prefer thicker wire. My soldering skills have gotten pretty good, so it's a pretty quick process to set up a new connector. If I had it to do again, I think I'd use 36 pin connectors... they're easier to find. I've looked for a system in the past with zero (or low) insertion force for the connectors, but an inexpensive and functional system hasn't presented itself.