Just saw it today. I find mirror universe episodes tiresome, but this was not bad. Giving it an 8.
I've probably said this before, but the phrase, "Terran Empire" originated in fanfic. The earliest usage I'm aware of is in Ruth Berman's short story, "Visit to a Weird Planet Revisited" (itself a parallel story to Jean Lorrah's "Visit to a Weird Planet"); Berman's story got promoted to "licensed professional" in Bantam's 1976 Star Trek: The New Voyages; Lorrah's is still pure fanfic (and I read it online some years ago).