We have all three and by far the one that gets the most play time is the XBox 360. Like a previous poster, we play a lot of Little Big Planet on our PS3. We have other PS3 games and they do get played but not all that often. When Little Big Planet 2 comes out in January I'm sure the PS3 will be on most of the time but until then we probably won't use it much.
We love the Wii, and it has great exclusive titles. The Mario games, Wii Sports and Animal Crossing are addictive. Right now I'm playing Epic Mickey and I love it. But the Wii's graphics aren't as good as the other systems and the online play is almost nonexistent, so anything that is available on multiple consoles we don't get for the Wii.
The 360 is our go-to system. Any game that is available on multiple consoles we buy for the 360. I like the user interface and the controller better on the 360 than the PS3. The 360 can stream Netflix, as can the PS3. With the recent update, the 360 can also stream ESPN content. Even though you have to buy a Gold membership to play online or use Netlix or ESPN, we still prefer XBox Live to Playstation Network. The most popular XBox games are the big first person shooters that primarily seem to appeal to boys and men, but there are lots of other great games available as well. The music, racing and
Lego games are especially good. (And Lego Pirates of the Caribbean was just announced - it should be really cute!)