For twelve stories over such a range of time and Trek series, they end up being remarkably similar. They showcase the violence and treachery of the Mirror Universe, all right, but not much more, and the stories themselves are uninspired. Also, a heck of a lot of established characters get killed off. I realise that part of the appeal of the Mirror Universe is that writers can raise drama with the genuine possibility of character death. By halfway through the book, known character deaths becomes routine. Drama is not found in routine.
The first four stories are sent in the Terran Empire. The first story is told in flashback, but you can't tell which point is the present for 'Trip' Tucker. The second story is an intricate story of betrayal. The third is James Kirk becoming captain of the I.S.S. Enterprise. It is good, but told in a creepily light-hearted manner. After these three good stories, the Vanguard story is basically an action set-piece. It is the kind of story that works better on a screen than in print.
After that, the stories are set in the post-Empire period, with a mix of Alliance and Terran Rebellion stories, with the Memory Omega conspiracy as a frequent sub-plot. Three stories feature the Stargazer crew, some of the Next Generation cast, and some of the Voyager crew, and all are basically about the groups treacherously decimating each other. Keith R.A. DeCandido's story is the mirror for the Battle of Marcan V. Part of the interest in the story is that you don't know who is treacherous or who they intend to betray. The other main part is that the writing is very good and, it has to be said, noticeably above the quality of the rest of the stories.
Peter David's story is what Star Trek: Nemesis would be if mirrored, using the Excalibur setting ... and a comedy. Or at least, I found it impossible to take the story seriously. That is especially a shame as it features one of the largest-scale events in the Mirror Universe.
Jim Johnson's A Terrible Beauty is about Keiko Ishikawa. It is an intricate story of decepetion and treachery and a very subtle style of establishing control over people and events, and one of the better stories. There is a story with the Titan crew that is basically the usual slaughter, plus since it's Chris Bennett, some heavy-handed moralising. The last story is a David Mack special ops story. It's not up to his usual standard.
Overall, these stories are decent, but nothing special.