Mirror Universe stories have to be the worst idea ever in Trek.
That's a bit much.
Apart from "Mirror, Mirror" and "Crossover", I would agree, at least TV-wise. The MU novels have been great, though.
I will agree that the novels have done a better job with the MU, but that kind of makes sense anyway. There's more room to build a story in a novel. You can get a little more hung up on the details than you can in a TV show or even a movie.