buying d20 only
I'm now buying only d20 books, etc. I have in my collection, GURPS, Shadowrun (1st ed), Teenagers from Outer Space, Call of Cthulhu (4th edition), Ars Magica (3rd edition), but you know what, none of them ever got played. With the arrival of CoC d20, one of the other players in my group actually volunteered to GM an adventure!
Supporting an open system also sits well with me from an ideological point of view. If other publishers don't understand the benefit of an open system where everyone gets to share the benefits of improvements, bug fixes, and a large, mutually compatible customer base, I do. My gaming time is precious, and now that I have a choice, I no longer want to spend it beta-testing for anybody else's "House System".
After all, I don't have to buy the Middle Earth Role Playing game, I can always crib of the net notes that so many others here contribute to. The community size of d20 is so large, that chances are, someone else has already done the work for you! When I first got started with the Temple of Elemental Evil conversion, I was daunted by the size of the project, but then I looked in the conversion library, and found a lot of existing monsters already converted! This saved me so much work --- I can't imagine doing the same conversion for say, GURPS, or any of Decipher's game, however elegant the game engine might be.
I'm not predicting the death of other game systems, but for those of us who have real jobs and a life, the time saving benefits of a large d20 community cannot be over-estimated.