What the hell is wrong with "The Emperor's New Cloak"? I just saw this episode for the first time and I LOVED it! I thought it was just as good as the other mirror episodes up to that point (I skipped "Resurrection" because the premise sounded lame and everyone said it sucked), and I was thoroughly entertained by it from start to finish. Mirror Ezri and Brunt were fascinating in how different they were from their counterparts, Rom's constant mirror world critiques were hilarious, it was cool to see Quark being brave, and I liked how Mirror Garak was as clueless and easily manipulated as the real Garak is brilliant and resourceful.
Also, the dual endings were both terrific. I loved Smiley's victory and the most blatant reference to homosexuality ever on Star Trek, which at the same time had some subtlety about it. Leeta and Ezri going off for some girl-on-girl and Rom's horrified reaction really cracked me up. I think it may even end up being my favourite mirror universe episode.
And I keep seeing people reference episodes that I really believe are totally watchable. Not great, but far from the worst of Star Trek. I actually really liked "Where Silence Has Lease" when I was younger. I think I've sort of grown out of it, but I wouldn't mind watching it if it was on TV.
It has some very (unintentionally) funny moments, like the creature (don't know how to spell his name) doing the 'kill the red shirt' Star Trek tradition and Picard's outraged 'WE CANNOT ALLOW YOU TO DO THAT!' (any time Patrick Stewart yells, it's gravy) and its fascination with Pulaski leading to the classic moment where sexual intercourse and the difference between women is explained to it and it asks someone to demonstrate that...by having sex. Pulaski's reaction is priceless.
I would classify episodes like that more as 'so bad it's good', as opposed to episodes like "And the Children Shall Lead" and "Plato's Stepchildren" where what happens is so completely devoid of humour, implausible, and boring or ugly to watch that you feel it insults your intelligence and wastes your time.