I look at everything, regardless of publisher. I buy what i think is good. Sometimes that means big-name publishers, sometimes it means somebody's first PDF product. Sometimes it means D20 System, more often it means something else. But, and i think this was the real point of the question, the presence or absence of a D20 System logo, or a D&D logo, doesn't really have any impact that i can discern on my purchases. Similarly, i don't automatically avoid any publisher's products. [I do automatically buy a couple publishers' products, or at least automatically if the topic interests me and i can afford it: Atlas Games, Wicked Dead Press, to name a couple.] Heck, even the game system doesn't necessarily cause me to pick up or avoid an RPG book, and all genre will do is adjust the purchasing timetable: a book for a genre i'm currently running will get picked up before one for a genre i'm not, but the latter will, at least theoretically, get bought eventually.
With one important caveat: there are some specific design choices in D&D3E that drive me batty, so i don't use the D&D3E core books. My default fantasy D20 System core books are Arcana Unearthed, Everquest D20, and probably soon Blue Rose. So, while i look at each WotC book that comes out, they almost inevitably don't appeal to me, precisely because of their tight compatibility with D&D3[.5]E. However, that's actually not a surprising thing, since i'd grown disenchanted with WotC's RPG products long before D&D3E came out. In fact, i haven't seen a WotC book that was worth the shelf space since they last owned Ars Magica or Talislanta (whichever was more recent). So it makes perfect sense that when they got a chance to redesign D&D, they'd do it in a way that didn't appeal to me. Not to the point where i don't even consider buying the books, but to the point where, despite giving them consideration, i've yet to actually do so.
[Of course, by "everything" i don't literally mean everything. First off, i'm sure there are entire companies i've never even heard of, especially in the PDF-only market. Secondly, i have to have some way to "look" at it--either it has to show up in the FLGS, or i have to find a preview or review, or something. Third, some things i can eliminate without even looking. I never so much as considered the stirge book, frex, because i've got no intention of ever using a stirge in a game, or even having them exist in my gameworld, so it doesn't matter how well written it is. Similarly, there's no point in me looking at a modern-military RPG, because i'm just not interested in the genre. ]
edit: Forgot to mention two things. First, my gaming buddies are all non-partisans, or anti-D20S-System partisans. Most buy whatever works for their game, and if that game is D20 System (as it often is), they'll use whatever books they think work best, which might or might not be D&D-branded, and might or might not be published by WotC. One, at least, will generally choose the WotC product over the D20 System product, if both cover the same topic. The other will, as near as i can tell, pick the better in that situation, regardless of publisher.
The other thing is that, IMHO, simply buying what i think is "best" has consistently driven me *away* from WotC products, even when there is a choice. So, frex, i *love* psionics in my D&D game. You couldn't pay me to use the Psionics Handbook. While the Expanded Psionics Handbook has apparently fixed some of my issues, and would be tolerable, i think the Psychic's Handbook is *much* better, and if i'm gonna put psionics into my D&D game, it'll be either that way, or with the Shadowforce Archer rules. Similarly, I don't think the new Manual of the Planes is anywhere near the best other-worlds book for D&D3E--in fact, i think every other one i've looked at is better [that'd be the Malhavoc book, and the Phil Reed book]. And i'm not yet convinced that any of them holds a candle to The Primal Order: Chessboards.
