Like all good Trek, "Nemesis" builds significant philosophical issues into the heart of the story. If Picard had lived the same life as his clone, would he have made the same decisions? If so, would they be excused by that life? No easy answers are given: the mystery of nature vs. nurture remains a mystery and is deepened rather than explained away. So far, very good.
On the down side, the movie is too derivative--too many scenes are too obviously ripped off from other movies: the phaser fights in the hallways from "Star Wars" (even down to jumping through the side panel!), the clone's death like Mordred's in "Excalibur" pulling himself toward Arthur along the impaling spear, Ryker kicking the Reman into the shaft like Kirk kicking Kruge off the cliff on the Genesis Planet, etc. etc. etc. And for old Trekkers, spending half a movie on Romulus without any attempt to dovetail Spock's unification movement or Yar's daughter into the plot is a bit disappointing. Picard was too involved in both of those story lines for them to be simply ignored in a Romulus-centered movie that may well have been the last chance to bring closure to them. But, then, the current generation of Trek producers seem to have forgotten about Continuity long ago.
Bottom line: B+ or B-, depending on which of the above paragraphs I just re-read. Worth seeing because of the first paragraph, but could have been much better.