Pacman for money moesn't interest me, I just can't see the longevity.
The games I've bought on Arcade are:
Marble Blast Ultra. Love this game! Recently gone back to it to try and get all the advanced levels in under par time, got about 10 to go. I also keep jumping on for a few minutes to try and improve any level and move up the boards.
Wik, Fable of Souls. Fun for a while, spoilt by no multiplayer. Still, I played it enough to get all bar one achievement and not much more to get the last one.
Zuma. Not bad but not yet sure if it was worth it.
Uno. I like this but only really played it twice (for about 5 hours one night)
Gauntlet. I got all the achievements but only because an old mate and I both remembered this from the arcades so spent an afternoon doing it. Put me off the whole retro XBLA thing.