This was a boring book that at best had okay moments.
Unfortunately, it was too wordy and the pace was too slow for my taste. Additionally, the ending was not very good either - in how they made it back "home" to their side of the alternate universe.
I never cared for "parallel" universe theories in Star Trek - or in any sci-fi for that matter - and maybe my pre-justice got the best of me here. I believe it to be a very outlandish concept. There is more of a chance that a "transporter" will turn out molecules into energy and then put as back exactly the same a long distance away - then for the theory of parallel universes to be true. Not to say that they can make a bad story, but the story must be done correctly. i.e. Q-Squared. The novel Dark Mirror, however, adds nothing to this concept that the TV shows already have. However, I suppose if you a fan of The Next Generation you never saw your favorite characters "mirrored", like the fans of The Original Series, Enterprise and Deep Space Nine. On that level, this might be for you.
Some of the story-lines were decent. For example, when the the crews (good and bad) met, was an interesting scene. I also appreciated the plot of having them sneaking aboard the "dark" Enterprise and replacing their "clones". Although, this was just a rehash of The Original Series episode Mirror, Mirror. The other highlight was the "dark" copy of Troi. Something I thought I'd never see.
One those things alone, I would not read this book because it took far too long to go anywhere.
The main problem with Dark Mirror, is that the author takes Star Trek way too seriously. It would have helped to add some humor here and there. I understand that it is a serious novel. but she made Picard and crew to be a bunch of humorless "sticklers"... something The Next Generation TV series already excelled at.
If you have something better to do (or read), I wouldn't waste your time with Dark Mirror.