I'll be honest: When I first heard "Enterprise" was planning to do a mirror universe episode, there was more than a little eye-rolling. I think DS9 went to that particular well a few too many time, leaving me more than a little sick of the MU.
Fortunately, we got something much more in line with the original "Mirror, Mirror", with some terrific callbacks to some other original elements as well:
When I heard they were planning to redo the "first contact" scene, I thought there was no way they could pull that off without James Cromwell. Admittedly, it would've been nice if the "Board the ship, take everything you can!" line had come from Cochrane himself, instead of that bearded dude, but it's a minor point.
(Alternatively, they could've brought back the young woman who played that Townsperson with the pretty eyes in "First Contact" and have HER say it, which would been even more twisted!
Life on the mirror NX-01 seems to be pretty consistent with what Kirk would encounter a century later, with the torturing of crew members and unrealistic number of mutiny attempts.
But what we remember, of course, from the episode is that lovingly-reproduced Connie Defiant
. IIRC, that was the initial concept for "Future Tense". While it would've been neat to see the prime universe crew reacting to this ship from the future, I think, in retrospect it was better used here: I don't really buy the idea that you can't let the prime-Archer see this ship from the future, given all the other sh!t from the future that he ended up seeing, but it would be almost impossible to come up with an excuse for him to make full use of the Defiant's power in the way that mirror-Archer did, let alone have the crew (especially T'Pol) put on some TOS-era uniforms.
It did occur to me that the ship's name never actually comes up once in Part I. It would've been a nice moment to have Archer read the name off the hull, or perhaps the dedication plaque, and a casual viewer not familiar with "The Tholian Web" might've been a bit confused, perhaps even mistaking it for the Enterprise, but it's not a big deal.
I am grateful that this episode effectively silenced critics from the early days of "Enterprise" who complained that the NX-class seemed more advanced than the Connie. Kind of hard to stick to that argument after seeing the Defiant blast the hell out of Tholians, Andorians, Vulcans, and finally the Avenger.
The CG Tholian and Gorn were all right. They weren't really what stuck with me from the episode, but frankly, it's hard to produce a CG/motion capture character that does, (other than Gollum). I would've have minded having the prime crew run into the Tholians again, however. They're a distinctly different foe than most of the other ones we've encountered, and, like the Andorians, it might've been nice to give them a few episodes to shine after being a peripheral race in Star Trek for so many years.
One minor gripe I had was with a couple of the characters introduced in the 2-parter, i.e. Admiral Black and the alien slave. There's nothing wrong with the performances (what's not to love about Gregory Itzin?) or even the characters themselves per se. It's just that we're been introduced to the mirror counterparts of characters we've never seen before. I can of a few other characters that could've filled the Black role: Admiral Leonard (the only one of the trio of office from "Broken Bow" who was never seen or mentioned again), Commander Williams, A.G. Robinson, or Captains Ramirez or Hernandez. There's probably one or two guest aliens they've encountered in past episodes who could've filled the role of the poor SOB who tells Archer there's a Gorn on board (Silik, perhaps?).
It's a pity they couldn't have stuck mirror-Shran somewhere on the Avenger too, but again, minor gripe.
The paranoid mirror-Archer's hallucinations of his prime counterpart were kind of fun (reminds me a bit of "The Alternative Factor", now that I think of it). We never really got to see too much in past episodes how mirror universe characters react to the prime universe (that's one thing I wish "Mirror, Mirror" had done more of), so that was fun. John Billingly's more sadistic Phlox worked really well too. (If you want to see him do an evil character REALLY well, check out the two episodes of "Cold Case" he did). Little moments like Malcolm twitching when Enterprise is destroyed also fit pretty well.
I don't know if I would've cared to revisit the mirror universe again (as I said, DS9 went to that well a few too many times), but as a stand alone 2-parter it was a great story from a great season.
Finally, how frickin' awesome was the opening sequence? Glad they took the time to do that. The mirror universe isn't one of Star Trek's more plausible scenarios (An empire where promotions come from mutiny and assassination would run out of capable command personnel pretty quickly, and the constant warfare makes it unlikely that the same individuals would be born, live at least long enough to meet someone and produce children at the exact same time as in the prime universe), but hell if it isn't fun (in small doses).