I walked into the theatre fully expecting the movie to be a case of: "Look! It's Kirk! And Spock! Ooh, and they're flying the Enterprise! Isn't that great?" But I was greatly relieved when it turned out the writers were serious and were actually writing a full-on star trek story.
MAJOR SPOILERS
The timeline split was, in my opinion, perfectly acceptable, and even kind of cool. The fact that we have familiar characters, but they are essentially new people(Considering that their histories and upbringing are different now), makes for very interesting sequel possibilities. I don't know if they should fix the timeline or not; Would it be better just to create this parallel universe and leave it at that? I don't know. I do know that everything in the movie may not have occurred at all in TOS(For example, the love between Spock and Uhura, or Scott being stranded on that ice planet), and that those little (Or big) changes drastically change the characters and environments.
Once Vulcan was destroyed, I thought: "Holy crap, Vulcan is gone!! That's horr--Oh wait. That doesn't happen in TOS. They'll fix that by the end of the movie." And I was surprised when they didn't. they had the balls to do something new, and I think that's great. But I guess you can never please everyone. Someone will find something wrong, and you'll never make the perfect movie.
I actually find it pretty funny that everyone got jipped in the parallel universe. Kirk lost his father, Spock his mother, Vulcan is gone, many ships were destroyed that didn't need to be, Spock Prime(Why is he called that, BTW?) wound up in the past, etc. The only one who got the good end was Pike; he's crippled, sure but not a vegetable! He got something good out os all that, ROFL!