The political statistics blog FiveThirtyEight expresses some scepticism about the value of Colin Powell's endorsement for Barack Obama, arguing that relying on endorsement represents a lazy short-cut for the voter and is therefore most effective when the election is unimportant, and least effective when most voters already have a vast amount of available information about the perceived strengths and weaknesses of the two main candidates.
There is also a question of timing. Endorsing someone who is way ahead in the polls looks more like jumping on the bandwagon, or a bid for a decent job after the election, than a serious attempt to swing opinion. However, endorsing someone too early in the campaign may be a tactical error as well. Note the critical importance to Obama of those senior Democrats who announced their support for him when the Obama-Clinton race was reaching its final stages.
I agree with FiveThirtyEight that it is irrational for a voter to rely on endorsements when there is so much at stake. But in this election, many voters will experience conflicting reasons to vote one way or the other, and this will include irrational loyalties to various groups. Some people have assumed that the military would automatically support McCain. Following General Powell's endorsement, military folk who were uneasy with McCain and Palin now have less reason to vote for McCain merely from some sense of group loyalty. But it would be insulting to the thoughtfulness and rationality of voters with a military background to suggest that many of them hadn't already come to their own conclusions about McCain's grasp of economics, or Governor Palin's readiness for high office. After all, the military have more reasons than most to vote for a Commander in Chief who will not make rash decisions.