My general take is that this is a solidly mid-grade Star Trek movie, rather of the same caliber as Star Trek III. It isn't one of the really great ones like II or IV, but it isn't terrible, either. It's certainly one of the better Next Gen movies. Not that that's saying much: Generations was dreadfully bad, and The Search for Shangri-La (or whatever the hell it was called) was turgid. This one's roughly as good as First Contact -- I don't think it's ever quite as good as the best parts of First Contact, but it's less uneven.
The pacing is sedate, to say the least, and I'm sure that's what turns some of the reviewers off. IMO, it's written rather like a good Next Gen episode, very internal and decently thoughtful. Indeed, while it does have its loud moments (big budget movies must blow things up), it's really an extended meditation on the subject of identity. As is typical of Next Gen, it hammers the point a little too hard, but does manage to be rather interesting. While I think the pacing is a little off, the dialog is unusually good for a Trek movie: it felt right for Next Gen and for the characters.
The tone is quite dark, much like the darker moments of First Contact. It isn't a fun film, but is pretty intriguing. And there's an overarching sense of melancholy, much like Star Trek VI -- you can feel, almost from the first moment, that this is the last Next Gen movie. Like that film, though, the melancholy doesn't get too badly in the way.
Overall, a solid B effort. Not one of the great Trek movies, but I felt it was worth the time and money to see in the theater...