All in all, this was a great one. Alternate realities and Star Trek have a real love/hate relationship. Sometimes they create some great drama – ie. “Mirror, Mirror,” “Twilight” and “City on the Edge of Forever.” Other times, it’s a real EPIC FAIL, as in “Storm Front” and “The Alternate Factor.” It’s been a while since I’ve seen the DS9 MU episodes, so I’m looking forward to those. And “Yesterday’s Enterprise.” Oh, and wasn’t there a movie or something that dealt with a weird alternate universe? I could have sworn I saw something like that . . .
Much as I love DS9, the MU is IMO its weakest part. They should have really stopped at the first one -
Crossover - when the MU was still scary and interesting, though I had problems with that one as well (namely, I am not buying that the Intendant is an alternate universe Kira Nerys - she just seems like a completely different character played by Nana Visitor; compare that to Mirror Spock - I am buying that Spock could turn out like that under different circumstances, in a universe such as the MU). The rest of them got sillier and sillier, it turned into an excuse for dressing up, overacting, silliness and girl on girl action - and don't get me started on the whole "evil lesbians and bisexuals" thing.
Originally, in TOS, the MU was interesting as it showed a dark version of our heroes, of Trek's Humanity, and of Federation - it introduced the idea that, under different circumstances, they could have turned out ruthless, cruel, barbaric, and a Human-lead galactic power could have been a conquering, oppressive empire. This is what makes TOS
Mirror, Mirror and ENT
In A Mirror Darkly great.
Crossover was somewhat interesting in that it subverted the optimistic ending of
Mirror Mirror, showing that Spock's rebellion might not have been a good thing for the Humans. But once the rebellion started, they should have just ended the MU storyline, as the MU had no point anymore. Humans were again the good guys fighting the evil Klingons and Cardassians. Well, how different and original.

I don't have to explain why that misses the whole point of the Mirror Universe, do I?
Trek literature offers a number of interesting alternate universes, though - check out the
Myriad Universes novellas, if you haven't already.