To qualify my previous remarks in this thread, I found the first DS9
one ("Crossover") to be the most palatable of all the ones from that series. IMO it had the essence of the mirror universe, without getting silly with it. But I found the subsequent outings to be wanting, culminating in the embarrassing 'return' of Bareil. Over the years the storytelling on DS9
had evolved in leaps and bounds, and it often felt (to me) that compared to the ongoing Dominion War arc, the mirror universe ones always felt like they were just... running on the spot.
The reason "In A Mirror, Darkly" on Enterprise
is so good is because, apart from its obvious fan service, it actually captures the mirror universe as seen in TOS. It compells the viewer the same way, hinting at an entire off-screen history that is tangibly different to the one we know. I don't often feel the DS9
episodes achieved that on a consistent basis. That and, as I say, the Dominion War arc was just plain more interesting