Depending on the book, sometimes I'm not really interested in reading reviews, but if it's a book I'm not certain about, I will read reviews from a variety of different sources (Amazon, LibraryThing, GoodReads, and BCF of course!) to get a better idea of what people did/didn't like about it.
When writing reviews myself, I always try to mention both good and bad (if any) aspects to give other readers a better overall picture of the book.
I find that people are all too willing to give negative reviews on the internet. It's one of the great (or bad?) things about it being anonymous. I've read some very harsh reviews.
I would definitely tell the publisher why I had trouble finishing it. This is, after all, the reason why they send the books out - to receive feedback. If they only ever received positive feedback and no constructive criticism, then they would get the wrong idea of what the buying/reading public actually likes, and chances are they'll keep publishing more substandard books and then wonder why they aren't successes.
That's what I always do! I would rather read about the problems that people had with the book. Sometimes they're problems I decide I can live with, and other times I might be put off the book because the problems described sound like they would frustrate me no end if I decided to read the book. That said, I think that ultimately the best judge of a book is yourself, and you shouldn't really let other people's reviews sway you either way. 'One man's trash is another man's treasure.'