I was at a bar in Boston over the weekend (actually it was
Kings Bowling) and they have a showcase-style machine with a 36" vertical monitor running what looked like a homebrew front-end and Frogger, Donkey Kong, Space Invaders, MsPac-Man (with speedup hack), 1942, BurgerTime, Galaga, Dig Dug and Rally-X.
The thing had a messed up 8-way joystick that sprung back too hard, so when you moved it forward and then centered it, it would always hit down- this made Frogger a complete pain in the arse and Donkey Kong was impossible.
One nice thing was that when you selected a game in the frontend (after inserting coins), the screen faded straight to the games title screen very smoothly. The atract mode fades in and out of each games attract mode- as if the games are constantly running behind the frontend.
There were people constantly playing it. I can't see Namco allowing people to put it's roms in a machine alongside Nintendo and Capcom games though, so I'm sure it's completely illegal.