Help! What to do about review requests!

I've seemed to find myself in an awkward situation.

I get a bunch of review requests from authors that are more unknown and have very small publishers. I only accept the ones that I genuinely think sound good. Lately I've been trying to get through more of these review books but found that some of them are bad. Really, really bad and I don't think I can get myself to finish it.

So what to do about it? The authors asked so politely for my honest review, though who really wants to get bad reviews? Especially if the reviewer had a hard time getting through the first few chapters. Do you stop reading and pretend the request never came in the hopes the author forgot, or do  you write the bad review and feel guilty?

Anyone else been in this situation? If so, what did you do?

Right now, I feel about as guilty as Denver: