Well, for a long time, I took a "look before you buy attitude" with Mongoose. Lots of people were down on them, but I had concluded that their good stuff was very good; eventually I identified the authors I could pretty much rely on to put out good stuff (Mearls, Younts, Hanrahan), and I sort of wrote off their problems to not being selective enough with authors.
Then, being the drow lover I am, I picked up Sheoloth without looking at it thoroughly beforehand.
My mistake. The NPCs in the book were... well, let's be frank, it was close to the shoddiest rules material I had ever seen (I'll give FFE's Green Races and Monstrous Wizards compendium the booby prize there). It should have never been allowed to see print in that format.
I went along quite happily using my "buying by author" philosophy, and it served me well for a while. Then Quintessential II happened. Younts made a valiant effort to work within the guidelines he was given, but the guidelines sort of sabotaged him. Mongoose's take on "substitution levels for mixed classes" was interesting, but ultimately, I feel it missed the mark.
Mongoose seems to have taken a philosophy of creating materials you wouldn't find in other books, and often reached out a little further with their rules variants. While in some cases, it has paid off, I think that there is no formula that will make this work consistently. That's why I feel that the Quint II series was a failure in my eyes.
I don't hold a lot of interest in Mongoose's OGL games, so can't comment too much there.
Mongoose still has a few diamonds in the rough, and Mongoose has expressed awareness of the problem and a desire to do something about it. I am finding some of their classic play books pretty good, especially the ones by Hanrahan (Planes and Dragons). Lizard is also an author that I think does a good job, and I have high hopes for Ultimate NPCs.