The production values were terrible. Many of the monsters were questionable, some where nearly useless, and a few were so downright silly that they seemed to make new monsters from April editions of Dragon to be downright serious minded. The really bad art didn't help the credibility of the monsters either.
Still, if you can get past that, it's got alot of cool concepts. On the whole, I'd say that it has stood up as well as the other two 1st edition monster manuals. Afterall, its not like it was a step down from MM1's production values, and its not like its got fewer interesting and useful monsters than MM2. The overall picture you get is of alot of really good ideas turned into monsters with greatly varying skill of execution. Or, occassionally you get extremely well executed monsters - like the Slaad - with very muddled conceptions.
I've used all of the following: Aarakocra, Apparition, Bat (Giant), Berbalang, Bullywug, Caryatid Column, Crypt Thing, Dark Creeper, Dark Stalker, Doombat, Elf (Drow), Ettercap, Forlarren, Gibberling, Grell, Hook Horror, Hornet (Giant), Huecuva (!!), Lizard King, Mephit (!!!), Scarecrow, Skeleton Warrior, Slaad (!!), Son of Kyuss, and Yellow Musk Creeper (and Zombie). That's not a bad haul AFAIC considering how picky I am about monsters. There are probably half as many again as I'd consider using but have never found a need for.