I liked Dark Mirror. It became non-canon when Dark Passions was released. Which in turn became non-canon when the current era of MU novels were released (I`m currently reading DS9`s Fearful Symmetry). I have enjoyed all the MU prose I have read (including those in the Shatnervse) and am looking forward to Soul Key as well as the 3 recent era MU trade-sized books and Rise Like Lions.
That doesn't make sense, considering that all the other MU books were based on the MU as depicted in DS9 (starting with Shatner's "Spectre" and Wright's "Dark Passion" duology in the books).
But if you watch "Paralellels", look at Data's explanation for Worf's condition. From there it can be theorized that "Dark Mirror" takes place in an alternate Mirror Universe that split a some point after "Mirror, Mirror", where in one universe the Terran Empire continued and in another it fell.