As good as it gets. Keeping the improvements from MM4 and adding some graphics improvements makes this a good game, at the least.
The stages are OK (Gravity man was really interesting with such a nice gimmick, loved it) but wave man is terrible!
The bosses are great as well, did not stumble upon any BS that annoyed me.
The fortresses were... Ok? Not really a lot to comment about them.
Overall, one of the greatest. Oh, the music, as always, was good!