From Time's Mark Halperin:
The decision is not only symbolic but, in terms of timing, one of great tactical importance. Powell is a brand unto himself in American politics, and clearly transcends the media's tendency to hype endorsements more than their actual importance to voters. However, the indisputable benefit that Powell brings Obama is that the former Secretary of State and general is sure to block out any chance McCain has of winning the next two or three days of news coverage, as the media swoons over the implications of the choice. It is simple political math: McCain has 15 days to close a substantial gap, and he will now lose at least one fifth of his total remaining time.