John Scalzi, who certainly has more experience with being asked "favors" than I do--strangers and would-be writers asking if we would read their books, introduce them to our agents and editors, etc.--has composed an amusing and strident rant on the topic. He is, of course, largely correct. Go read it, especially if I don't know you and you're thinking of asking me one of those favors.
Then read his follow-up, in which he admits that of course sometimes he says yes to those requests (as have I)--that the issue isn't in the asking but in how you react to the answer, whatever it may be.
Also a good policy on writing blurbs, which I think I will quite possibly adopt.
Edited later in the day, because this seems to be the topic going around in the community of writers at the moment.
This is just to add links to Lee Goldberg's horror story, and to a video of Harlan Ellison doing a dramatic reading inspired by Josh Olson's original essay on this very subject (but from a Hollywood perspective). I haven't had any experiences like Lee's--mostly what I get is silence from the other end, whether I do the favor or not. Once I was unfriended on MySpace by someone who wrote a manuscript that I couldn't blurb because I genuinely did not care for it at all. No one has ever completely gone off on me, but there's always a first time, I suppose...