I'd guess Powell helps marginally with military types and moderate Republicans, and he'll rob McCain of at least two news cycles, which are a precious commodity at this point. But, overall, he's not changing the dynamic of the race so much as affirming it. The question is, does McCain interpret it that way? Does this help McCain see the writing that's probably been on the wall for two weeks? And, if so, will we start to see McCain campaign the way he wants to be remembered, rather than in a way that maximizes resentment toward the likely next president?
A lot of people will criticize Powell for jumping on Obama's bandwagon. But his endorsement may be more about what comes after Election Day than the election itself. If Powell brings about a change in tone that leaves the country less divided, he'll have performed a real public service.