I just wrote a nice, long review for this...and it disappeared.
I will just summarize what I had written previously.
Holden is a difficult character. He is difficult to like because he tends to get himself into situations...and complain about them. He is difficult to hate because he has that innocence and vulnerability and fear that many of us have felt, or at least understood. He is difficult to sympathize with and difficult to respect, but is someone who needs both sympathy and respect. He is immature, but knows and sees more than many adults do.
I don't think that Holden is a brat. He is lost and confused. He doesn't understand life. There have been two events in his past that have made him: the death of his brother and the suicide of a classmate. His problem is his attitude. Some people are bothered by the fact that he is pathetic, selfish, and insecure at the beginning of the novel and pathetic, selfish, and insecure at the end. But it's not as though he doesn't change. It's not as though he doesn't learn anything.
To have him have learned his lesson like a good little boy and earn a Boy Scout patch for good citizenship would be unrealistic. That's not the way the real world operates. Some things about some people will never change, or will take more than one experience and a few days to change. Holden may not have completely altered his view of the world, but he altered it at least enough to redeem something of himself. In fact, to have completely changed his vier of the phony world into a sunny, happy place would be just as bad, the opposite extreme.
But Holden does begin to see, at least somewhat, by the end, that others have emotions and complex feelings beneath their possibly 'phony' exterior, or whatever group Holden may categorize them in. For example, after leaving Mr. Antolini's Holden feels guilty because he knows he was too harsh and too hasty in his judgment. He can't dismiss Mr. Antolini as a 'flit' because he know better, he knows Mr. Antolini is a deeper person than that. When he lets Phoebe on the carosaul (sp?) reach for the gold ring because he sees that she must--all kids must--and if she falls down, then she has fallen, and nothing can be done. But she at least has to try. Everyone has to grow up. Some people will grow up twisted or screwed up, but that can't be helped. Growing up is a part of life that must be accepted.
He also changes his attitude of towards the world by at least deciding to try harder at school and being more accepting of his position in life. He knows that not everyone is phony--like the nuns, Phoebe, Jane--and has already at least tried to be more open to others. The reason many people identify with him could be because everyone has shared some of the feelings that Holden has felt, whether it be lost, confused, scared, anger, depression, pessimism or any of the wide range of other emotions he has experienced. For most people, puberty is a complicated time. It's not always a very pretty time. The transition from childhood to adulthood is difficult for some people, because it means losing their innocence and entering a world more polluted with deceit and lies.
Holden's experiences epitomize what many teens (especially of the time and era) felt as they approached adulthood. If any era was the era of phoniness, it was the Fifties, what with Marilyn Monroe, teen rebellion, a booming movie industry, and other fads that weren't 'real' they were just there to take up space and time, to entertain but not really enhance. They were, to some at least, cultural fillers, ritzy glitzy stuff that didn't really equate with great art and literature.
Holden represents, in many ways, all teens and what they have to go through to grow up and accept the adult world. That's my two cents.