Yeah, for my money I don't think Diane Duane ever wrote anything less then a great star trek book.
Dark Mirror is lots of fun. The standout for me has nothing to do with the mirror universe plotline though, it's Hwii. Duane is brilliant at creating memorable characters, and memorable alien species(although from memory I think hwii was originally supposed to be from earth? not sure on that).
Also great to see a more confident Barclay again!
Felt a bit of deja vu talking about this, did a search and found out why:
Looks like we already have a review thread for this.