On the other hand, “Keep on the Borderlands” is a better introductory adventure than “Castle Mistamere”.
The advantage of Mistamere is that it starts the party in the dungeon (well, just outside it). The DM doesn’t have to read pages of text about how many pieces of silver the tailor has hidden in a sock under his mattress. He goes straight into, “You see a crumbling castle; there are doors lying on the ground in front of it. A monster attacks!”
The advantage of Keep is that the DM is expected to use his brain, and figure things out. He must pay attention to the map key, and figure out how long it will take the adventuring party to reach the caves of chaos, and whether or not the party will get lost on the way there. It sets up an expectation that the players have options. Maybe they take out the monsters in the valley piecemeal, but maybe they head south into the swamps, instead, or maybe they try to loot the keep, and steal everyone’s socks.