I like Through the Looking Glass. To me, the µ-verse started great on DS9 and lost quality with each episode. Crossover was great, TtLG was good, Shattered Mirror was okay, Resurrection was poor and The Emperor's New Cloak was unbelievably awful.
Yeah that's pretty much how I see it. Each time they went to the Mirror Universe well, they came back with something of lesser quality.
I think that Through The Looking Glass is fun. It's certainly lost some of the nuance from Crossover, but the MU is still an interesting place for me.