Just to clarify my stance - I am not biased against WotC.
The question that I ask, whether 3rd party or WotC is 'Will I use this in my games?' I tend to be slow in regards to making a decision, circling like a vulture over a dying cow, and only making a purchase when I am sure.
Most of the material by WotC does not fit the flavor of my games. This is not a good/bad judgement, but rather a flavor issue. It can be wonderful, but if I do not feel it will fit I ignore it. That said, I tend to treat D20/OGL material as a buffet, picking items from a lot of sources, item by item rather than source by source. So, while there is much of Heroes of Battle that I will not use there was enough for it to be a worthwhile purchase (though I found it a bit dear).
The Mongoose Quints usually have enough material to be worthwhile, often the side materials, such as the rules for libraries in the Quint. Wizard book, are what sell the book to me. I found enough material in D20 Deadlands that while I will never use it in its own right I do not entirely regret the purchase. The original splatbooks have enough to keep me happy in that same regard, if I had not purchased them then I would probably have bought the Complete books, in this case WotC is competing with their own earlier releases - too much overlapping material.
Critter books... I prefer to tailor to a setting, ditto for feats and PrCs. Again a flavor issue rather than a good/bad judgement. So I will buy a Monsternomicon over a Monster Manual 2 every time.
Stormwrack covers concepts that I use in every campaign, so while I fully expect to ignore 75% of the material there may well be enough to make it a good purchase. By contrast I greatly prefered Frost & Fur to Frostburn, but while I have not yet purchased Frostburn I may well decide to purchase it when/if I run an arctic game.
That said, I do think that the D20 license helps WotC, if it were not for third party support I would have dropped the game years ago, it is all the cooks adding to the buffet that make the meal worth eating. And one or two purchases a year from WotC is better than none.
The Auld Grump