I've played both ACX and Armored Core - FF, and I have to say that both were a pretty awesome waste of my homework time. Seriously, I think ACX rivals AC6 as my favorite game ever, and AC-FF has killed plenty of hours trying to build the best robots to fight your way to the top of the league.
Either one of the Wipeout games is good for racing, but that's about it. Still fun, though. The difficulty spike between speed levels is higher than you think.
I've also never played fl0w, but I hear it's a head trip. Same with Loco Roco, except it's more dizzying.
Of course, I'm not aware of the prices on these games, so they may vary.