I didn't mind it. The Siskos are a failure, but at least they fail in a blandly likeable manner that doesn't kill the episode. "Bland" is unfortunately the keynote for the episode, though, as the production team continue their mission to make the mirror universe indistinguishable from all other Star Trek.
Let's begin with the plot, because I could use a laugh. The mirror universe rebels have taken control of Terok Nor, thus bringing the status quo nearly in line with our reality. Wherever you are, Deep Space Nine is in the hands of the good guys. Alas, though, there's no guarantee that they'll live long enough to enjoy this, since the Alliance is sending an entire fleet to squish them, led by none other than the Klingon Emperor, Worf. (Sorry, "regent of the Klingon-Cardassian Alliance". No, this isn't implausible at all. All positions of importance in the mirror universe must inevitably be filled by people who happen to be series regulars in ours, ahem.)
Anyway, our mirror-heroes want to be able to fight back! They could do this if they had a Defiant, which it would seem is a super-ship capable of fighting off the Alliance fleet almost single-handedly if captained by (tadah!) Sisko. Coo. Tough little bird. Apparently in the regular universe it's an experimental vessel, built to fight the Borg. Anyway, what do you think our mirror-heroes do?
(a) Infiltrate our universe and kidnap the real O'Brien, so that Smiley can steal our Defiant. Sisko must go and steal it back! (Wrong: too interesting.)
(b) Build an exact copy of the Defiant, despite the fact that mirror-humans had been until recently a slave race without any education save what they'd managed to scrape together. (I guess Jennifer Sisko helped a lot.) They also did this with limited resources, having only recently captured Deep Space Nine and obviously not having the support of Starfleet. Nonetheless they succeed in doing all this (um), only to discover that their battleship has structural problems. They thus kidnap the real O'Brien to fix them. (Wrong: O'Brien isn't the hero of the show.)
(c) Okay, right, they kidnap Sisko. This must be so that he can pilot it... except that it's not, because at the end everyone assumes he's going home, including Sisko, and the mirror-heroes are astonished when he stays for the shooting. They wanted Sisko strictly in an engineering capacity. You know, because he's an engineer and the station's Chief of Operations. Ahem.
(Sisko even tries to take O'Brien with him! Nope, sorry. "They weren't invited.")
No, the thinking behind this episode is "wouldn't it be nice if we brought back the mirror universe's Jennifer Sisko for a three-way reunion with her non-husband and non-son?" That's the episode's emotional core, or at least it would have been had the actors been capable of playing it. Felecia M. Bell and Cirroc Lofton are both almost hilariously incapable of the emotions you'd expect in this situation, but on the upside they're both likeable and naturalistic. You're happy enough to be watching them. They're not painful. They're just pointless. Bell's less bad than Lofton, by the way.
As for Avery Brooks, this episode improved my opinion of him. I now think he's merely bad. His (abysmal) Shatnerisms in the earlier mirror universe episodes were clearly deliberate, because he's stopped doing them and instead is just doing his regular performance. In other words, he's now trying to do Patrick Stewart. This explains so much. He's not doing Stewart's accent, obviously, but he's attempting the same level of elocution and the same sub-Shakespearian delivery. It's just that he can't make it work and the attempt is killing almost all naturalism in his performance. Nonetheless, after allowing for this, Brooks is clearly a better actor than Bell and Lofton and capable of reaching levels they're not. It's just that that's still inadequate, mostly obviously in the attempt at an emotional finale, in which we learn that gunshot victims will survive as long as is dramatically convenient.
I didn't hate their scenes, though. They're not achieving five per cent of the power they should have had, but at least the rest of the drama doesn't depend on them. They're just there. Every so often the three Siskos will pop up and be nice at each other, with the odd mild exception when it's just mum and dad. Besides, it's possible to empathise a bit with Bell in her pleasant way. Our introduction to them is startling, incidentally. Jennifer just pops up on Deep Space Nine as if one's mirror counterparts regularly drop in for coffee, while Brooks's beard briefly made me think he was playing his mirror counterpart. (I presume I wouldn't have thought that had I been watching other Deep Space Nine episodes.)
The best thing about the episode, as always, is mirror-Kira. She's in the hands of her enemies and she'll have to make dramatic decisions, but she's still the same purring, hypersexed bitch queen from hell. She's cool and she's funny. I loved her response to "you sentenced my wife to death", for instance. She's also murderous enough that it's a bit silly when she doesn't kill Jake, the real reason of course being that you can't kill a non-mirror series regular.
I also liked Nog's motivation, which made me laugh a lot. (I'm not talking there about liking tall women, although that's understandable since his head's at their boob height.) Incidentally, I see that the Deep Space Nine mirror universe episodes are currently three out of three for Ferengi-killing. They want to kill someone, but they don't want to kill anyone important and/or pretty!
I was less impressed with the Worf-Garak scenes. They're fine, but they're panto villains shouting at each other in a little hived-off corner of the plot that doesn't affect anyone else. They're amusing, though, e.g. Worf's improvement on an emetic.
It's okay. It's once again gutting the mirror universe even worse than previously and its central three-way relationship is bland and badly acted, but it's a passable time-waster. Its bad bits aren't killing the good stuff, of which there's a fair amount. It's worth it just for Nana Visitor, who's mighty. Colm Meaney is fun, in a deadpan sardonic way. I don't mind the Dorn-Robinson double act. There's even a spaceship battle at the end as if they're trying to be Star Wars, which in fairness is quite well done even if spaceship battles inherently aren't that interesting. It's reasonable telly.