I didn't mind the big rewriting, they where part of the plot. For Star Trek, a time travel plot is almost cliche really. But I'm sure the movie studio saw it as a benefit, since they can now easily retread the material of the original series without recreating it in exact detail. They captured the character of the original characters very well. In particular Bones is brilliant in his short bits. The new Kirk is intentionally more angry and angsty then the old one, even more of an unstable rebel when pushed. Spock is fine, Uhura decent, Scotty a bit over the top, but so was the original. The Spock/Uhura relationship seems to be another bit of intentional alteration. Chekov and Sulu OK, though Sulu seems to be using more traditional sword fighting then fencing.
What was more jarring where the little, pointless stuff. The off hand references to races and knowledge that Star Fleet, in the original series, had not yet discovered yet. The set design, which didn't really share anything with the original. The villains ship was particularly odd, in that it defied the internal logic of ship design in Star Trek. And the amazing efficiency of the teleporters, which suddenly where working better then those of the Next Generation. Plus, why the radical change in the special effect of the teleporters? It was one of the iconic bits from the series, and the new effect is no better then the old one.
On the technical side, the cutting of the fights scenes was poor in places. Forcing all of the characters into this one plot stretched things, but that was inevitable, for it to work they had to include all of the major original characters and make sure they all had a scene. Most of the writing was decently clever, working in the characters signature lines without being forced. There was a nice balance of serious and humor, spaced out to hit when it was actually unexpected.
The plot held together well. Both the hero's and villain's goals and methods make sense. They use the tools they have available, and they do it in ways that makes sense for them. Which is to say the villains plan isn't particularly rational, but for a Romulan, throwing everything away for a chance at vengeance is not surprising.
Overall, I would say not quite as good as the Wrath of Khan, but probably the second best movie so far.
Jay