LBJ gets a bad rap that I believe is mostly undeserved. He did his best with what Kennedy started: the bad (Vietnam) and the good (Civil Rights). He didn't even run for a second term. Compared to Nixon he was Honest Abe.
LBJ was responsible for the escalation of the Vietnam War, transforming it from an ill-conceived venture into a major foreign policy, and a full scale war. It was largely due to his conviction in the 'Domino Theory' that perpetuated that meat-grinder that his support was so low before the 1968 election. I will give him that his support for civil rights was probably part of it as well. At any rate, it's not my sticker. :)
I think we also have to give him points for the fact that he was never really meant to be President, and he had to serve right after Kennedy. It would be hard for anyone to be remembered super favourably when they had to follow after one of the most charismatic politicians in history.