I had an Xbox 360, and loved every minute of it. The game selection (for my tastes) was amazing. I'm mostly into shooters/action games. The graphics are stunning (espeially in HD with surround sound).
Xbox Live/marketplace made up for about 95% of my usage. Online multiplayer games, downloadable content (demos, videos, gamer pics, etc, etc). Xbox Live is a well-structured online community that actually works really, really well. Despite the fact it costs $60 for 13-months of service, it's worth every penny.
Unfortunately I sold it to pay for rent.
When I get enough cash to buy a next-gen console, I'm going with the PS3 so I can experience that. Mostly for Killzone2 (was a big fan of the first and Liberations for PSP). Not to mention some of the PS3-only games like Resistance 1/2. Fallout 3 is great on it too.
The Wii, I hate it, and would hope it died in a chemical fire. If you're into family-orientated/non-violent games (yes, I know there are some violent games for it), then go for this. I hate the motion sensors and the WiiMote/Nunchuck.. As intuitive as it is, I can't stand it. The last thing I want to do when I'm in a "session" is to stand up and wave my arms around like an idiot. I just want to sit down, chill out, and play some video games. That's just my opinion, hope this helps!