I do play and enjoy d20 RPGs other than D&D: Spycraft, Star Wars, (the d20 version of) Call of Cthulhu, Deeds Not Worlds.
With regard to D&D, I am not one to confine myself to WotC's output by intention. I do so, for the most part, because I attempt to confine myself to products that I know that I shall use often enough to make the purchase worthwhile. This is why, at the present, I intend to do no more than purchase a copy of Unearthed Arcana, The Player's Guide to Faerun, Faith & Pantheons (maybe some other Realms products) and The Book of Vile Darkness so far as WotC products are concerned.
This takes me to the third-party publishers. I stick to Green Ronin Publishing, Atlas Games, Malhavoc Press, Sword & Sorcery Studios & Necromancer Games for about 80% of my third-party purchases at present; if I can get an active D&D game going in the near future--my Spycraft game is stalled due to a player-player fight, and I may have a Star Wars game going soon--I will see if I can convince them to go with either Midnight or Dawnforge because I like both settings, but right now I have no justification for buying either of them. (Otherwise I'll go with Freeport, Greyhawk, the Realms or Rokugan.)
I am not interested in products obstensably meant for use with D&D that inflict radical and counter-productive changes upon it. As much as possible, I want to retain D&D's great strength of being able to "play it dumb" (as Bruce Baugh put it); to just jump in and know what's what with little more than the three-paragraph opening crawl of the Star Wars films.