Some people like it, but my basic complaint with it is that it introduces a new concept, the "patch", that is basically a crippled commit. When a patch is not a commit, you get rejected diff hunks when it fails to apply, and it can easily forget its history.

An MQ patch is basically a commit that doesn't know how to merge and doesn't keep backups. It's way too easy to make a mistake and lose work. I consider MQ one of hg's youthful mistakes before tools like histedit, rebase, and Evolve came to exist.