For me, it's a little like when I remember a good meal I've had. The best meals are the ones that I've eaten and then still feel good about afterwards and that I'd love to have again. Sometimes, I find, that I'll enjoy a meal while I'm eating it, but then feel horrible afterwards because there was so much fat in it or because it gives me heartburn.
I feel the same way about books. There are some that I enjoy while I'm reading and that I still enjoy afterwards and feel good about. But, there are others that, though I enjoy them while I'm reading them, afterwards I don't feel as good about them and which I can't recommend whole heartedly.
By the way, I will be posting a review about the book that made us laugh (How do you tuck in a Superhero?) at the end of the month =).