"Mirror, Mirror" is one of the all-time most popular Original 'Star Trek' episodes, and for a great many years was my personal favourite 'Trek' episode. In many respects it still is, and either way defiantly makes it into my Top 5.
I've always loves stories of parallel universes and evil twins, and this story does it very well.
We had already had glimpses of evil counterparts, such as Kirk's brutal double in the first season's "The Enemy Within" (which was also caused by a transporter malfunction), and in examples such as "The Alternative Factor", but this is the first time the notion of a parallel universe is fully explored.
Parallel Spock, who isn't far off from his 'good' self, sports a goatee beard, starting a trend for 'evil twins' that would be duplicated many times (one of my early television memories is Michael Knight's evil (sort of) brother Garthe sporting such a goatee in 'Knight Rider's 'Goliath').
The parallel costumes and make-up are good, with almost a pirate-y feel (I have to say, Uhura looks pretty hot in her parallel costume), and there are other nice touches – for example, Sulu has a huge scar running much of his face, no doubt the result of some previous attempt to better his rank.
The only weak element in an otherwise great story, is Kirk's romance with Marlena (Barbara Luna). I find the scenes extremely flat and dragged out, and there is very little real spark between the pair. It is the only flaw in an otherwise excellent episode; I would have much rather seen a bit more of the evil Kirk and co. in our universe instead.
Other than that, this is an excellent story, with some great touches, and very re-watchable. The "mirror world" concept would be continued with later 'Star Trek' series, (beginning with 'The Next Generation's third season episode "Yesterday's Enterprise"), but as good as some of those offerings are, I still find the original the best in many ways.
All-in-all, a great episode, and deservedly one of the most popular of the Original Series. It still stands as one of my all-time favourites and definitely in my Top 5 – possibly my single favourite episode of the series.