Wii and PS3 - plus gaming PC, which was important factor in my choices as noted below.
When 360 launched I already had a gaming PC and like Xbox expected to get most 360 centric titles on PC. Certainly all Valve stuff, plus at that time it was known Gears would head to PC and both Halo and Halo 2 had also released on PC (where I prefer to play them). Therefore, for me specifically, the 360 didn't represent much value in terms of getting access to games as at launch only a few titles really were full 360 exclusives and they didn't warrant the purchase for me. I'm happy with this choice as pretty much every big 360 game I'd want has landed on PC such as Mass Effect, Bioshock and Oblicion (although many of these also eventually landed on PS3 of course) and really only Halo 3 and Gears 2 remain outstanding and I'm not going to get a console just for two games. I'd like Alan Wake, too, but it's short and offline so I just borrowed a friend's 360 for that. I still figure it might hit PC, too, although as I've played it now I probably wouldn't double dip.
Wii I got due to the hype and the lure of the controls. Happy with it but only really use it with Nintendo releases. It's never felt like a console I could own without other gaming machines, but it's great fun - particlularly with the family gaming - and at launch the only place to play such games (still is and will be until Move / Kinect launch).
PS3 I got because pretty much all its exclusives clearly weren't going to land on PC and where first party there was no chance they were timed (as many 360 exclusives felt like and subsequently proved to be). It also had more titles I was interested in (not at launch but upcoming). I also wanted to get a Blu Ray player and so that was a good part of the package. I figured PS3 and 360 would end up with almost identical libraries regarding multi-plat titles due to their similar specs and this has panned out as I expected, so a few exclusives aside I really don't feel I'm missing anything by not having a 360 at this point.
So for me the PC, PS3 & Wii combination is optimal. I get access to the most titles this way, missing out on only a few that I feel doesn't justify any additional expenditure on a 360 (although if Kinect takes off I will very likey buy a 360 for that in the same manner as I got the Wii because I love to try new stuff).
I do like the 360 having played on it a lot at friends, and I particularly like the new model - much better IMHO and had that been the launch model I'd have found the choice between PS3 and 360 a bit tougher - although I'd probably have stuck with PS3 on the library/access front. It's just that for me, already having a gaming PC when 360 launched and a strong focus on PC / Steam for online the 360 simply didn't offer much - although I can see that if I had the same focus on FPS/online but no PC I'd have bought a 360 very quickly.