With Deep Space Nine, the mirror universe pretty much became the vehicle in which characters who died in the regular Trek universe (lest contractual issues get in the way) could reappear for whatever MacGuffin they happen to be searching for, though the episode featuring on the Ferengi and Rom's constant proclamation of "it's alternate" (or words to that effect) was just utterly annoying. On the flip side, it was also the vehicle in which the writers could kill off regular characters without having an impact on the regular universe.
With Enterprise, it wasn't so much that it was a view of the Enterprise-era mirror universe, but classic Trek elements with a Constitution class ship and the retro (and neo-retro) sets.
Other than that, I think the original "Mirror Mirror" episode was the best of the bunch. The DS9 episodes, with the exception of "The Emperor's New Cloak" were okay, but where the hell was Odo's (or even Intendant Kira's) goatee in "Crossover"?
The preceding post is OK to print.