I loved Dark Mirror. I also wish they had done a MU TNG. Then we probably would have had something similar to Dark Mirror rather than the fallen Terran Empire ruled over by the Alliance.
Security Officer Deanna Troi was an amazing character, and I think that she would have translated well to the smell screen.
With some friends, a couple of weeks ago I watched the sixth-season episode where Troi is forced to pose as a Tal Sh'iar agent ("Clearly, your father was a traitor."). I mourned, just a little bit, for the episode that never got made.
Dark Mirror's impact for me lay in the way that it made so many could-be possibilities of TNG a reality--Picard and Beverly are a couple! and so are Riker and Troi! and Troi's a stronger telepath, confident in her abilities! and Wesley's a go-getter!--and make them go so horribly wrong.