My buying experience with Mongoose is limited to the past 12-18 months, so I can't speak to the products the company produced in its early years. However, I have yet to be disappointed by a single Mongoose purchase that I have made. A couple of points/opinions I'd like to make:
1. The editing issues vary from book to book. Some are very tight, with few or no errors. Others have more than their fair share. However, I didn't find them to be worse than I saw in prior years from Traveller, Rolemaster, and White Wolf products. Not that they're desireable, I'm just surprised at how every negative Mongoose post seems to focus on this aspect over content.
2. Mongoose has a very wide range of products. Just like WotC, every line/book is not going to be fit everyone's taste. I have little interest in playing core-rulebook-style D&D (it still has too many of the sacred cows that caused me to abandon the earlier editions) so things like the Quint. series haven't earned my dollar. On the flip side, I am extremely interested in their OGL and licensed games (OGL Cybernet, etc.; Conan; eyeing Lone Wolf). Regardless, I researched Conan and OGL Cybernet before I bought them. Do the same with any book and your chances of feeling burned decrease significantly. People who have purchased 1 or 2 books sight unseen and then write a company off don't come across as knowledgable or objective to me.
3. Mongoose's Conan line is top-notch. I find it to be a better set of rules than the D&D core rules. It's supplements are original, detailed, and inventive and incorporate the right level of PG-13 & R-rated material to convey Conan without going over the top. They aren't the sanitized Conan-in-name-only version TSR did (and WotC would likely do today).
4. By way of comparison, I've been burned by Swords & Sorcery to the point I usually have to physically look at one of their books 2-3 times before I'll "risk" my money on their products. I purchased a significant amount of Scarred Lands material and was amazed and disgusted at how unbalanced their d20 crunch was. Talk about your Munchkin's Dream. Also, the quality of the line was very inconsistent, which seems to be a criticism of the Quintessential line. My point here is that without a tight thematic focus such as a licensed-product or genre focus, you're going to get varying quality when different authors write different books in the series. Look at Scarred Lands. Look at the Quintessential series. Hell, look at the Complete <flavor of the month> books from AD&D 2e. This may be a fair criticism of Mongoose's other lines, I can't say. I can say, though, it is a criticism that equally applies to other companies.
Finally, Mongoose is, by all acounts I can find, a profitable d20/OGL company that:
A. Is growing where many of the d20 companies are shrinking or on life support
B. Admits mistakes and tries to address them to keep their customers happy. Most companies try to argue away criticisms of their products, dismiss them, or ignore them. Mongoose will step up and say, "Yes we've come to realize there is an issue with ..." and then tries to address it. Do you think WotC is going to say, "yeah, we really dropped the ball with d20 Future but we weren't willing to risk the money needed to do the sci-fi genre right."?
C. Is earning MORE of my RPG dollars than any other company.
Just my 2 cents.