I, too, have been asked to weigh in on this at various places. Of course, the predictable problem is that they never ask commentators to distinguish between political analysis and policy analysis. The two are often quite different -- in fact, they are often complete opposites. For example, I would have told you that Bill Clinton was a great politician -- but a bad president.
Similarly, if you were asking me to judge Obama's style and adeptness at public relations -- and the skillfulness in which he has sold his message to the public these first 100 days -- I would likely give him at least a solid 'B'.
On the other hand, if you were to ask me about his policy -- I would tell you that his support of a $787 billion spending bill, not to mention his attempts to nationalize health care and seize control of private companies, earned him an 'F-'.
As such, I've settled on assigning him a 'C' -- which is, I think, generous.