When "Mirror, Mirror" first aired in 1967, who would have thought we'd have to wait twenty seven years for a sequel?
Unlike sister show TNG, DS9 is quite comfortable from the get-go acknowledging the original Star Trek stories, and this episode kicks off a total of five Deep Space Nine episodes set in the mirror universe. "Crossover," a Kira episode to say the least, might be the best, with the cast (much like Shatner & company) having a blast with their new roles. (As Kira says, "The players are all the same, but everyone seems to be playing different parts") The fun spills into the episode, and the result is simultaneously an homage to the past and a foundation for the future.
Director David Livingston deserves a special award for "best direction of an actor in a dual role", as no other Star Trek episode doubles up a character so believably. It's easy to watch the two Kiras in "Crossover" and forget that there weren't two Nana Visitors on set.
It's just too bad that the episode's ending is a foregone conclusion from the start!