This is going to sound insipid, but dungeons are great when they are good. Good dungeon design is difficult. Dungeons should be there to be explored and there should be plenty of informed choices made. A cthedral with a cellar that leads to a crypt where the mummy lives ins't a dungeon -- it is a linear set of encounters the PCs are forced to go through, in sequence, if they are to succeed. A good dungeon has discreet areas, multiple entrances and exits, "unimportant" levels, empty rooms, interesting puzzles. Moreover, a good dungeon requires the players to think strategically about resources -- everything from time to rations to hit point to spells -- and force them to make aformentioned informed decisions.
Too many dungeons end up being strings of encounters that serve no purpose aside from burning time until the final encounter. If that's what you're going to do, you might as well just spend 30 seconds telling the players what they fought through to get to the Big Bad and then run that fight. The dungeon should rather be the adventure -- with a good dungeon, you don't even need a Big bad or even a "story" or plot. One of the things I love about 1E is that characters were assumed to be adventuring because they were adventurers. There didn't need to be a threat to cosmic security for them to take up sword and spell and explore labrynthine underground dwellings -- just the promie of great risk and reward.