Okay, I guess that I finished the story arc yesterday because I closed the portal to the mirror universe, so I can judge it.
First of all I'm a big fan of the original TOS mirror universe, not so much of what DS9 made of it. The first problem with the story arc is that I already met the Terran Starfleet in three deep space encounters and two random missions long before I got the first mission relating to it.
The second problem is a lack of background. I really would've liked James O'Brien, Ja'rod and B'Vat giving more information about what happened in the mirror universe.
The third problem is that except James O'Brien they all look like Starfleet officers down to the delta shields on their chests. That makes sense for those who infiltrated DS9, but the others? You have the mirror universe symbol, sashes, mini-skirts and knee-high boots in the character creator. Use them!
The fourth and last problem is something that's maybe a bit difficult to do from a technical point of view, but it would be really, really awesome if you would meet the mirror versions of your captain and his bridge crew.