I've never head anything but hate for this module, but after reading it I think the haters are wrong. It has an interesting ecology, and well-developed new monster species (which has since been reprinted in a monster manual, but it was a good addition to the game at the time). The intrigue between the flame salamanders and the bat-people is fun, as is the murder plot by the rival adventurers.
I plan to use the loose, somewhat free-form encounters in my present underdark campaign. I'm pleased with this adventure, especially at today's prices.