Honestly, to those that like to pretend they're not playing d20 when they play d20 variants like True20, Iron Heroes, etc... come off it already. You're playing d20... it might be variant or rules-light, but the core of the system is d20. I don't mean to upset anyone by saying this, just one of those things I find annoying.
That said, I've played a lot of d20 and variants (D&D, d20 Modern, Spycraft, Iron Heroes, Arcana Evolved). I think d20 has served my gaming needs well, and I could see myself playing nothing else and being very happy.
I've also played some other systems that I rather enjoyed (Traveller, HeroQuest, Exalted), but none of them would serve me as well as d20 and its variants. Like Ryan noted, I just want to play. Learning a new system is interesting and fun, but playing is a gazillion times better.
--sam