For me once I move past the cover and the book synopsis and begin to read I like to be drawn into the book and its character by the first half a dozen chapters if not sooner. I love when I am reading a book and I want to keep reading to see what is going to happen next. That the book is so good that I hate to put it down and when I do I can't wait to go back to it.
As for not liking a book I can't say that I have picked up a book and not like it but I tend to stay in my comfort zone of reading which is Vampires. Now there have been time when reading a book or book series that I had to walk away for a bit but not becasue I didn't like it but becasue I was burnt out on the series or just wasn't feeling the book at the time.
How about you at what point do you decide if you like a book or not?