Unlike almost every other facet of the show, DS9's mirror universe episodes got worse during the series' run and were pretty weak at the end. But this first episode is exceptionally good - it goes beyond the mere gimmick of "similar but different" to offer something genuinely substantial.
There's humor here - Bashir hitting on Kira, Mirror Garak admiring tailoring, Odo exploding. And the actors (particularly Nana Visitor) have great fun playing against type.
But really, it's the climax at the party, where Smiley O'Brien gives his impassioned speech, that makes the episode. That's what I meant when I said there was substance here. Colm Meaney rocks.