I remember buying my 360 for Halo 3, like the majority of 360 owners. Thats all I bought it for, one game, it had nothing else that was good. I would say the online is better, but is a bunch of screaming 12 year olds REALLY better?
I bought my PS3 because although it didn't have much games, the games it does have are all unique and really fun to play, like Uncharted Drakes Fortune, Heavenly Sword, Warhawk, Resistance: Fall of Man, Ratchet and Clank Future: Tools of Destruction, and Moterstorm. unlike the massive amounts of gritty/realistic FPS titles for the 360... seriously, I am really getting tired of seeing games where the main color is gray and/or brown.
as for the Wii, the only reason I can see myself owning one is for Brawl, which isn't even that fun.