Boogie Nights is nothing like Breakin 2: Electric Boogaloo

I've seen this movie numerous times, it was really hard for me to detach from the movie to see the script on it's own, much like it was with Pulp Fiction. Regardless, I really enjoyed reading it. Something occured to me while I was reading it though. I've read a number of books prior to seeing the movie version, either because I had been putting off reading the book and the movie coming out motivated to read it, or just because I had read the book previously. Whatever the case was, after having read and enjoyed the book, I was disappointed in the movies. This is one of the reasons I have yet to read any of the Harry Potter books. To be fair, I haven't watched the movie version of books I didn't like, or conversely read the books that movies I didn't like were based on, however I haven't found this to be the case when reading a screenplay. I've been able to keep them distinct enough that if something was left out or changed, that I liked I wasn't dissatisfied with the movie. With that in mind here are my discussion questions...

1. Was anyone able to follow my line of thought and find themselves in the same situation or conversely find that they were disappointed in a movie after having read the screenplay?
2. Is it possible to have a crappy screenplay but have it turn into a good movie?