Looked at it because it promised vampires. Of course, my definition of vampire being Anne Rice's, it didn't offer quite that, but then again I didn't expect it to either.
"I Am Legend" is odd for horror. Normally horror has absolutely no point, leaves you with exactly nothing except maybe a little shaking and a few nightmares, which is why I don't read it. This is different. It has a message, and the end is worth everything...
The ten short stories that follow it in the book are rather regular horror. No message, and sometimes not even scary... So I will have to say I'd give "I Am Legend" an 8, but "officially" I have to rate the whole book, so...