I am building a vertical cocktail machine to run just the older games like pacman, frogger, etc. I decided I didn't want o mess with the GPIO pins, so I went with the I-PAC VE. The VE is the value addition, which is $37 shipped and it pretty much plug and play. The only downside to the VE is that you are stuck with the standard MAME button configuration (i.e. it can't be reprogrammed like the more expensive versions) which is perfectly fine for my needs.

Since the I-PAC VE outputs keystrokes, it works without configuring anything. In fact, I tried to use the controller configuration tool in PiPLAY and just got myself all messed up.

A few things I've learned that are useful to know.

Holding down the player 1 start button acts as a "shift" key for the I-PAC VE allowing you to access a few critical keys without adding more buttons. Works out like this:
player 1 start+ player 2 start = ESC
player 1 start+Left = Enter
player 1 start+Right = Tab

With those 3 key strokes, you do most anything you need to in PiPLAY or MAME

However, there are a few issues with the PiPLAY Menu that can cause a problem or are just not intuitive.

In the PiPLAY Menu keys 1 and 2 are mapped to the configuration menu and quitting PiPLAY. Unfortunately they are the keystrokes outputted by player 1 start and player 2 start. Which can cause issues for someone not familiar with the setup.
Also, having to hold the player 1 start and moving the joystick to the left to select an option isn't very intuitive.

So what I wanted was to disable the player 1 start and player 2 start in the PiPLAY Menu and have the "Fire" button = Enter.

Fortunately, this is fairly easy to do. you'll need to edit controller.yaml file. This is located in /home/pi/pimame/pimame-menu.
You'll need to change 3 things:

1. Comment out the line as below to disable the 2 as a quit button
BACK:
- pygame.K_ESCAPE
#- pygame.K_2

2. Comment out the line as below to disable the 1 to access the configuration menu
MENU:
- pygame.K_TAB
#- pygame.K_1

3. Add the last line below where shown to make the "Fire" button (the Left Control button) act as the select button
SELECT:
- pygame.K_RETURN
- pygame.K_SPACE
- pygame.K_KP_ENTER
- pygame.K_LCTRL

One last note using the I-PAC VE. If you do happen to end up exited out to the terminal by accident, you don't have to breakout the keyboard. Linux has the cool little feature built in that by pressing the up arrow, you can toggle through the last commands used. So assuming you recently used the commands say: exit, sudo reboot, or sudo shutdown -h now, push your joystick up until you get to the command you want. Then hold the player 1 start button down and push the joystick to the left (remember, that equals enter) and the command will be executed.