While there has certainly been some excellent material produced under the OGL my impression is that it was outweighed by large quantities of low quality, poorly written rubbish which glutted the market. It made picking out the useful from the dross pretty damn difficult.
I know certain parties have argued that the Bad-OGL outweitghed the Good-OGL but my own experience doesn't bear that out. And with the number of reviews available on the web, I never felt I was an uninformed consumer when purchasing a 3rd-party product.
For every Book of Erotic Fantasy (which I think was d20 license, not OGL), I can site multiples that were top notch products, such as Mutants & Masterminds, Conan OGL, True 20, Game of Thrones, etc. Also none of those products diminished my WotC purchases - it increased them due to the fact that I could plug-n-play so much of it.
I think the volume/glut of available product can't be disputed. I think the "most of it was bad" argument is demonstrably false. YMMV.
Tangent #1: I also find the volume/glut as justification for moving away from the OGL particularly funny given that many people trumpeted 4e as being a smart move b/c WotC had "printed every viable subject they could for 3.x" (I'm paraphrasing , obviously).
It still cracks me up that people can make a case that something is bad for the hobby when a 3PP does it but the exact same reason/statement is good for the hobby when WotC does it. It's even funnier to me when the argument for both cases is made within the same post.
Tangent #2: I also love the "WotC is right to go with the GSL b/c OGL sales compete with core rulebook sales" and the "3PPs sales are barely blips on WotC's/D&D's radar". Again, often argued by the same person in a single post. Either the 3PPs sales were strong enough that WotC felt the GSL was necessary to defend against it, or the 3PPs sales were small enough that there was little business justification for going with the GSL just because the could - i.e. the "Might Makes Right" issue. It's unlikely (but not impossible) that both are true.
As a wise man once told me, "Never underestimate the stupidity of a corporation." For grins, he also said, "the only thing that can exceed the stupidity of a corporation is the stupidity of government" and "the only thing that can exceed that is the stupidity of academia".