I own a PS3 and a Nintendo Wii, nothing against the XBox 360 (having the three seems a bit extreme). There are plenty of very good games out there for the Nintendo Wii, games for the casual gamer and the hardcore gamer. Here's a list of the games I own to serve as an example:
1. Super Mario Galaxy (a bit kiddie but a very fun and high quality game)
2. Mario Kart Wii (very fun multiplayer and great online support)
3. No More Heroes (for the hardcore gamer, and cooler than most games out there)
4. Metroid Prime 3 (simply beautiful, for hardcore gamers) Just for the record, Samus Aran as a movie character is way more interesting than Master Chief.
5. Madden 09 All Play (not better graphics that the PS3 or 360 versions but it's more fun, and with enough depth to please a football fan)
6. Pro Evolution Soccer 2008 (a game that lives up to its name, a revolution for soccer videogames due to the great control scheme that's impossible to emulate right now on the other sistems)
7. Wii Fit (come on, it's the only way a hardcore gamer could do some workout)
8. Guitar Hero World Tour and Guitar Hero III (loads of fun at parties and the like)
9. World Of Goo (that's an instant classic and shows that the WiiWare games have potential)
And the upcoming games do look promising (The Conduit, MadWorld, No More Heroes 2, and more).
I think the Nintendo Wii is the easy target for hardcore gamers due to its lack of graphical power compared to the other consoles. But there are fun games, and not just for casual gamers.