It's the first Deep Space Nine mirror universe episode. It's okay.
Firstly, some background. The mirror universe has been seen in a Classic Trek episode (Mirror, Mirror), five Deep Space Nine episodes and an Enterprise two-parter (In a Mirror, Darkly) that's set before all the others.
What's different about the DS9 episodes is that they're post-Empire. Mirror, Mirror and In a Mirror, Darkly were set in an alternate universe where the equivalent of Starfleet was the evil Terran Empire. These stories are brilliant. These guys were like scum squared. Tyranny was official government policy. Promotion was earned by assassination and showing mercy in a position of power was a crime punishable by death.
However things changed after Mirror, Mirror. Captain Kirk inspired mirror-Spock to reform the Terran Empire and make it less dictatorial, which apparently caused the Empire to be overthrown by a Klingon-Cardassian Alliance. These guys aren't nice either. Humans are now slaves and Deep Space Nine is a place of summary executions.
I have mixed feelings about this. On the one hand, progress is good. I applaud the writers for acknowledging change and not simply trying to clone to the past. However the Alliance is less fun than the Empire, because they're less evil. They're brutal and they've enslaved an entire species, yes, but they're merely bastards rather than complete and utter backstabbing horrendous planet-shattering bastards. Part of the difference, I think, is that the Empire was more dynamic. It was always looking for more planets to conquer and enslave, better weapons to exterminate populations with and so on.
The Alliance, on the other hand, are just sitting there. Intendant Kira Nerys rules Deep Space Nine and she's basically happy with life. She doesn't even really like killing and will often stop her sidekick Garak from going too far. As for the humans, they're either slaves or jumped-up slaves who've been given special dispensation. Let's do an evil headcount, shall we?
Kira = a bit evil, sometimes, but not too much.
Garak = evil.
Odo = brutal, but that's all we see.
Quark = mirror-Quark is brave and noble!
Sisko = not a particularly nice guy, but he'll discover his conscience.
O'Brien = slave.
I'll ignore the non-mirror Kira and Dr Bashir, who've crossed into the mirror universe through a wormhole.
That's not enough evil. It's okay, but it's nowhere near enough to hit the "you must be joking" level of the Terran Empire. One could argue that the status quo we see here is justifiable payback. Besides, having humans enslaved by aliens makes the episode feel cosier and less challenging. Humans as the bad guys is a cool reversal of expectations, especially if we're the mega-super-bad guys. Humans as the victims of aliens is the same premise as gazillions of SF adventure stories and it's not challenging any of the audience's assumptions of moral superiority.
The alt-universe set-up is thus less attention-grabbing. Despite this, though, what's good about the episode?
Nana Visitor is having fun with her two roles, being sexy, childish and dangerous as mirror-Kira. The joke of Quark being the altruist is quite funny. The episode isn't afraid to kill regular characters, because of course they're not the regulars but instead their mirror counterparts. Um...
Is that really it? I suppose so. I didn't hate the episode, though. It's fine. I quite like Kira's relationship with mirror-Kira and the general tone of still-moderately-evil. Oh, and I was mildly amused by the opening scene of Dr Bashir annoying Kira.
On the downside, I didn't believe all of it. Apparently the mirror universe techs have redesigned their teleporters to ensure that universe-crossing could never happen again. Gosh, that's clever. They have a complete understanding of multiverse-spanning temporal physics, do they? Ahem. I also didn't buy mirror-O'Brien deciding at the end not to go back to our universe with Bashir and Kira. Let me think about this. He can either: (a) go back to a world where he knows someone like him can be successful and live a happy, contented life without being beaten or enslaved, or (b) go on the run from the all-powerful alien slaver empire with a guy we saw bullying him.
Overall, it's okay. It's not too bad. It's watchable. It's even been listed as one of "Ten Essential Episodes" from Star Trek: Deep Space Nine in a book called Star Trek 101. It's a bit glib, e.g. everyone being casually au fait with parallel universes ("on my side"), but it deserves praise for pushing forwards with the mirror universe storyline and I'll be seeing how it continues.