Odd as it sounds The Quintessential Fighter was where I first started liking Mongoose. I wanted a mass combat system, and there it was. And it worked!
There were some balance issues, fighter schools needed a price to pay beyond money (the answer that both Mongoose and I came up with separately was an experience point cost). In fact I liked most of the Quints. And Seas of Blood remains the only D20 supplement that I have ever worn out. (It was loved to death, and died happy.) I liked the fact that the books went in directions that most companies weren't going, even when they felt like house rules that had been written down.
Yes, they are a bit amateurish, but I look back at some of my old favorite gaming supplements and can't say that they were any less so (Thieve's World by Chaosium, Free City of Haven by Gamelords, City State of the Invincible Overlord by Judges Guild... my old favorites) in short they hit a nostalgia button.
Lately I have been running an OGL Steampunk game. Goodness! it has editing problems. I had to rework a great deal of the material, but still feel that it is a good game, just one that could have benefited from another six months of development time. Having tinkered with it I like it fine, but then I view the D20 system as a salad bar, and as time passes wish that I had more items as PDFs, just to make cannibalizing systems easier...
The B5 material has so far had okay editing, the second release of Conan seems fine, but I think a lot of people will be going over the editing in Mongoose books with a fine tooth comb, as they are now suspect.
The Auld Grump