I agree wholeheartedly about Viewtiful JOE!!!!! Unbelievably fun game.
I used to play my GCN way more than my Xbox because there are far more deeeeeeep single player titles on it (e.g., Metroid Prime, Zelda, SMS, Skies of Arcadia), but have been playing my Xbox like crazy now that I have Xbox Live
It's really fun to play against others, but for single player immersion, I usually go to my cube. That changed when KOTOR came out and will probably change when I get the GTA 2 pack, but for now, I'm doing single player stuff on GCN and multiplayer on the Xbox.