What is Nick's opinion of Tom and Daisy at the begining of the book? How does it change?

At the beginning of the novel, Nick does not really know Daisy and Tom well.  He mentions in chapter 1 that "Daisy was [his] second cousin once removed and [he'd] known Tom in college.  And just after the war [he] had spent two days with them in Chicago."  Nothing about these meetings tells the reader that Nick had any previous depth of feeling toward the Buchanans.  

However, when Nick has his first dinner with the Buchanans in Chapter 1, the reader understands Nick's feelings better.  First, when he describes Tom, he notes, "I always had the impression that he approved of me and wanted me to like him with some harsh, defiant wistfulness of his own."  Nick seems to understand that Tom may appear gruff and cruel on the outside, but there is something in human nature that causes people to want others to like them.  

As for Daisy, Nick seems infatuated with her; he describes her voice: 

It was the kind of voice that the ear follows up and down, as if each speech is an arrangement of notes that will never be played again. Her face was sad and lovely with bright things in it, bright eyes and a bright passionate mouth, but there was an excitement in her voice that men who had cared for her found difficult to forget: a singing compulsion, a whispered “Listen,” a promise that she had done gay, exciting things just a while since and that there were gay, exciting things hovering in the next hour.

Daisy is the type of woman that men are magnetically attracted to, even Nick.  One could say that he is mesmerized.

However, all of these feelings change at the end of the book, after all the tragedy that occurs at the end.  Nick blames Tom for Gatsby death and blames Daisy for not being strong enough to make a decision.  One of the most famous quotes of the book talks specifically about Nick's feelings:

They were careless people, Tom and Daisy — they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made. . . .

And that is what Nick was left to do, clean up the mess that Tom and Daisy left in their wake.  And Nick "couldn't forgive [Tom] or like him" for what he had done to Gatsby, even though at the beginning Nick's feelings for Gatsby were more harsh than at the end, a reversal of his feelings for Tom and Daisy.