I buy WoTC D&D products almost exclusively, though I do supplement them with a few d20/OGL adventures from the DCC line (and soon, the Gamemaster/Pathfinder lines). Why? Well, for one thing, it is easier to find WoTC products; no matter where I go, at least one of the chains will have WoTC products. Second, I find it easier to convince other people to use WoTC material (for many of the reasons stated on this thread). Third, I love much of the WoTC IP and wish to see it incorporated into my games. Fourth, I do like WoTC production values (if only their editing would get better). Fifth, the design philosophy behind the current WoTC products meshes well with what I want out of a D&D game. And lastly, I really love the inter-product support that WoTC has introduced into their materials; if I love the warlock from Complete Arcane (and I do), I want to see it supported in other supplements, adventures, on the WoTC websites, etc.