I hated Mike's Rust Monster... Mostly because it's the one lower level creature that can CRIPPLE a 3E character. If something kills you. Meh. You can be rezzed. But your equipment? Man, that's a low blow... So I take it upon myself to put FEAR into the players by applying templates to the Rust Monster to beef it up. (The Anthropomorphic half-red dragon one is just evil.. especially as a Rogue)
Oh, and I wouldn't count on Mind Flayer being in MM1. If next year is psionics, they should be in MM2. Plus, we were told that some traditional critters for the first book wouldn't be in it from the beginning....
Chris