They're all great in their own way but if you have or plan on getting an HD tv and were to pick one system for the long run....you're getting the best value with a PS3. Very high reliability rate for hardware thus far, free online service that is growing, good games on the horizon, wireless keyboard/mouse support, a web browser, etc. It's expensive because they packed it chock full of capabilities from the start. The media makes it sound like it's doing horrible but it's actually selling comparably to what the 360 did its first year. Their main concern is the lack of software so far.
The 360 has the better multi platform titles thus far, and a more expansive online service for $50/yr. I've heard that MS may make a new version with an internal HD-DVD drive because of their current standard dvd drive's limitations. That would effectively screw over anyone who's gotten a 360 up to this point, so it's up to you if you want to get one now or wait.
Nintendo has a unique controller but a lot of developers are wondering what the future of it is as far as creative content. It's the cheapest of the three but has a disappointing selection of games other than the mascot titles from Nintendo themselves (Mario, Zelda, Metroid, etc.). The new Mario game looks fun as hell; makes me want a Wii for that alone.
All in all though Sony took a gamble with a videogame system/BluRay player/media computer but they're building steam little by little. If the system was going to fail, it would have already. Its main detriment is the titles take longer to program for, which many developers have complained about in the past. They said the same thing about the PS2 though and that system is still selling, and better than the PS3 at that.
I sold my Xbox 360 after I beat Gears of War and got a PS3 shortly afterwards. Its functionality has kept me entertained enough with the lack of AAA software titles. They'll have more of those in the next few months though so it was a good decision on my part.