Not sure why I dislike this episode. It's dark, but so are other episodes that I do enjoy.
Despite the mind control angle, this is really just another "terrorists hijack an airplane/ship" story. The Enterprise leadership attempts several gambits to regain control, and the terrorists parry those. Aside from the relatively slow introduction, the plot proceeds at a brisk pace with few if any dead spots.
Maybe it's the "evil Data" and "evil O'Brien" that throw me off. If you don't have the same gut reaction as I do, you will like this one a lot, because it's an exciting 45 minutes.