There is a short, very interesting, and fairly negative review by none other than Mike Mearls over on RPGNet (
http://www.rpg.net/news+reviews/reviews/rev_1250.html). It's well worth reading -- if only for the funny parts. I am not sure how serious Mike was here, or if he would even agree with his own words completely today (it appears to have been written back in 1999).
I think one thing that he really bangs on about this module is actually a great virtue: the utterly generic nature of the setting. There's no detailed backstory, none of the NPCs have names.
To me, I think this is an incredible virtue. I have run the Keep on the Borderlands about half a dozen times: in Greyhawk (twice), Mystara (twice), the Forgetten Realms, and in a low-magic homebrew setting. Because there were so few details, it was trivial to move the module into any of these settings. For example, the Shrine of Chaos has been variously a temple to Tharizdun, Erythnul, Moander, and Baphomet.
Around 1999, I found most published modules had far, far too much detail -- and were therefore of rather limited value. I have a large gaming group with a diverse mix of role-playing and combat; I found most of the background fluff was utterly useless in play: it was too tangential to the storyline to ever get used. And much of the background that wasn't tangenial needed to be stripped out or heavily modified in order to fit my campaign.
For what it's worth, Dungeon magazine seems to have found a real sweet spot in terms of background information. More often than not, there's enough info to pique my interest and sustain the adventure, but not a lot of excess material that needs to be stripped out, modified, or ignored.
I do recognize many of the serious logic and continuity problems with this module, but all-in-all, I find it to be a pretty good introductory adventure that new players still seem to enjoy.