Let me try to de-partisan this a little bit.
It's understandable that he harbors such resentment to those who held him captive. But having said that, it's also an inexcusable use of a racist term by a prominent politician who should know much much better.
It seems many of you jumped on one side or another without reading everything.
Not only did he specify his captors, he also apologized and said he would not use the term again.
Now, I wouldn't vote for McCain in a million years anyway, but I don't see this as something that would really affect someone's vote.
As Cawacko noted, you all have reasons to find this offensive or not offensive. The people who would vote for McCain anyway are just the kind of people who would see absolutely no problem with his insulting his captors (no matter tha language) and those who would vote against him are the type who would be inclined to see it as evidence of deeper racism.
Basically this is not going to change any minds about McCain one way or the other.