IAMD was better than the entire MU arc on DS9 in my opinion.
I agree on that one.
DS9 >>> ENT
DS9 MU <<< ENT MU
I think I probably disagree.
In a Mirror Darkly was entertaining enough, but I liked
Crossover and
Through the Looking Glass better. My problem with
IaMD is that, although the first half was good fun, there really isn't much of an entertaining story in part 2. I thought it ended up relying on camp and nostalgia and not much else by the end.
Not sure what you mean by no story - there's the failed rebellion by T'Pol and Soval, and the battle for power of the Terran Empire between the Humans. I don't see DS9 relied on 'camp' any less than ENT MU episodes - I'd say it relied more so, even if we leave out "The Emperor's New Cloak", what with the Intendant's characterization and costumes and the overacting of Farrell and Siddig as their MU characters.
Re-watching
In A Mirror, Darkly, I've started to think I might like it even more than
Mirror, Mirror. I like that it's 100% a Mirror Universe episode, even down to the new opening credits and the
awesome theme, with no characters from the prime universe and fully from the POV of the MU people, and that it's so bleak (which, of course, it had to be, due to the time period, since we knew that the Terran Empire would only get stronger from that point on).
Mirror, Mirror and
Crossover, when you think about it, fit the same format: our heroes cross over to the parallel universe, are shocked at finding out about their 'evil' counterparts, and before they leave they affect the people in the MU universe and inspire them to rebel, and it ends with a message of hope that things are about to get better and the oppressors will be overturned or at least challenged.
In A Mirror, Darkly features people from the MU finding out about the Prime Universe, but what they find (The Defiant) only helps the Terran Empire become stronger, and it serves the most ruthless people go their way to the top. It does also inspire some people (T'Pol) to rebellion against the Empire, but the rebels (the only faction that we could feel some sympathy for) meet a sorry end - with just a prophetic line by T'Pol to suggest that the Terran Empire would fall one day - and the battle for power between Humans is all about who can be the most ruthless and scheming. Which all makes it the darkest of the MU episodes.