Bought and played Mario Kart today myself. The online playing is pretty good, though I sucked at it. Played the single player version and unlocked a few things, its an overall solid game.
GTAIV I will get this week, but I'm in no rush. I have still 5 unopened games for the 360/Wii sitting here gathering dust. It took me 3+ months to open Burnout Paradise alone, which is quite a fun game IMO.