probably my favorite module, i don't even remember all the times i have played it, and whether we finished or not it was always fun. that may have to do with the the fact that my dad-the dm- likes to add all kinds of his own craziness into published adventures, so each time he did something else different. i've run it a few times too. it is a relatively easy adventure to run without much prep and is good for introducing people to the game.
Literally played the cover to pieces on this one ... bought another copy a couple years back, and kept right on playing it.
B2 has many warts and flaws of course, but I think the thing that has made it so great in the many campaigns where I've used it is KotB's familiarity.
I've simply spent so much time in that gully that I know what's happening there any time of day. I've skinned and reskinned it and dealt with each of the weird gaps of logic in several different ways. I've DMed it as an abandoned silver mine, a leprosaria, and a set of sea caves where religious fanatics held sway over a most unusual flock. I've DMed it as a prelude to Keep on the Shadowfell as the goblins, under the oversight of evil clerics, dug frantically to find the Rod of Ruin so that Orcus might be summoned to Earth. I've used parts of it in other systems (Vampire: Dark Ages) as well as in gimmick campaigns (all PCs = wizards). I've set it in a desert and I've spread the caves out so that they were separated by miles. I've made the Castellan the true villain and other times left the keep decimated by pox and fever.
I think for guys who've run this puppy 8 or 10 or 12 times they must be able to animate the setting and the critters in the same way that Ed Greenwood does Forgotten Realms.
B2 is like an artist's palette, a squish of ochre here, a dab of vermillion over there ... the various adventures we craft using it are the paintings.