As the first module for Basic D&D, Search of the Unknown has a history that makes it very appealing to a DM. If you're looking for a very old-school dungeon, this is it.
As an artifact of its time, B1 is an excellent read for any DM looking to become more literate on "how things used to be." The front matter contains helpful advice on running old-school games they way they were intended to be played.
As a dungeon, there are many cool and interesting locations in B1. However, there is also an awful lot of empty space and boring rooms with the simple purpose of housing a random monster. Why the monsters would be in these empty rooms is beyond me.
When I ran B1, I had to do it after some heavy modification. Essentially, I removed any location that didn't have something interesting or fun. While this resulted in a dungeon about half the size, it did provide an adventure that was packed with great high-fantasy magic and fun.