After writing the below commmentary, I realized I generalize the comments as applying to all D&D/d20/OGL products. I realized that adventures are a different story, and rather than edit large sections of the post, I putting this caveat in the opening.
----
I still buy WotC books. In fact, right now almost all of my D&D/d20 purchases are WotC. Part of that is because of Eberron. The setting was made for me. That's not all of it though.
In another thread I was commenting about how I thought that Magic of Faerun was the best WotC product produced for 3.0. I went over the 3.0 non-adventure products in my head and I realized that aren't that many 3.0 products I consider top of the line (the core books aside). 3.5 has many products I feel are top quality and often innovative.
Third party D&D/d20/OGL products, on the other hand, are finding far much less use. In the 3.0 days the d20 market was a mishmash. Most products didn't work well with other companies products, and occasionally not with other products from the same company. Quality was all over the place, there was some drek, some great products, and a huge mass of products with good ideas but poor execution.
Today the quality is higher as far as printed products go (the PDF market still resembles the early years of d20). However, the products are more specialized. They are typically designed to work with a specific campaign world or else are "replacements." If they are campaign world specific, they require a lot of work and waste a lot of space if you don't play in that campaign world. A lot of Monster books are the most usable in this category, but typically have from 1/4 to 1/2 the content beings so camapgin feel specific that I won't use them.
The "replacements" are another matter. More and more products seem to be focused at replacing parts of the D&D system. Don't like the D&D magic system? Here is a system to replace it. Don't like the psionics system? Here is something to replace it. Few products (outside of adventures) seem to be designed to work with D&D.
I really don't want to replace the D&D system. If I don't feel like using the D&D class system I don't look for a point buy system to replace it. If I want a point buy system I'll play Fantasy Hero. If I want a less tactical game I'll play Heroquest rather than find an alternate D&D system that's less tactical.