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.
mq keeps its patches as an internal per-repo stack. This is much weaker than git's tree branches.
Here's an example: let's say I've been working on a repo, and have 2 current mq patches called A and B pushed. There's no way for me to pop A and B, then apply another patch called D in the current repo (at least as far as I could figure out). Also, even if there were, I'd have no way of then pushing A and B on top of D. The patches only work as a linear stack (D must come on top of B).
It's a very powerful tool, but also a complex one, and one which brings in a bunch of new commands. There are a bunch of extension which provide nice UIs to various subsets of mq's full power, and are much simpler.
If you need mq, use mq, but if you just need to e.g. do some history edition, use histedit.
