I wanted to link to Andrew Wheeler's musings on writing negative reviews. I'm facing something similar currently, so it definitely spoke to me.
One of my favorite things about this reviewing thing is that I've gotten to meet so many wonderful people. Among them have been many authors. But when you meet a wonderful person, and then read their not-quite-so-awesome book, it sure makes it much harder to write the review.
Well, that's why we get paid the big bucks... oh wait.