I'm quite the gamer, so I usually buy most of the systems just so I can play the best games.
Last generation, I bought the Gamecube for Mario Kart, Sunshine and of course the Star Wars flight titles. That's pretty much all I ever played on it.
My PS2 is used exclusively for DDR, Katamari and Karaoke Revolution titles (I just pre-ordered the newest DDR for it to get the "free" soundtrack CD.
My Xbox was my favorite of the last generation, mostly due to Halo and Halo 2, though PGR and Soul Calibur were also given countless hours. Worms 3D was also a sleeper hit for me.
I've owned Every version of the Gameboy since the original black and white one, except for the Micro. I went right to the DS and the DS-Lite from there. Tetris and Mario Kart DS are both awesome, New Super Mario is amazing.
This generation, I already have my 360. The other two, I have a feeling I'll get eventually, though not next month.
At over 600 dollars after tax before you even buy a game, the PS3 needs to show me some completely wonderful titles before I'll make that kind of Neo Geo type investment on it. I've seen some good demos, but I want to see actual games. They've been pretty quiet lately. I didn't pre-order, so I think this might be a 2007 console....if the price goes down or they release some awesome games. At launch, it's just a lot of money to spend on a Blu-Ray player I don't need.
I think I'll end up with a Wii, though I'm not sure if that'll be at Christmas time or sometime after. There's a great video on IGN where they walk you through several of the titles. The graphics suck, but the console looks like it might be fun to have around for parties or whatever. I'll probably use it exclusively for that, as I can't imagine waving my hands around on a regular basis just to play a game.
Today though, it's pretty much all about the 360 and DS. I'm playing Dead Rising right now, along with Ninety Nine Nights and a few of the arcade titles on Xbox Live. When those are done, I'm getting Saints Row (too many good reviews from my friends not to get it). In the next few months, Gears of war, DOA Extreme 2, Viva Pinata, DDR Universe, Lego Star Wars 2 and a bunch of others will be competing for my video game dollar. My thumbs are getting sore just thinking about all these good games.