The best way to learn is from feedback by a produced screenwriter. Just using 'bad' scripts without knowing why they are 'bad' is like trying to learn to drive a car by looking at all the accident reports. On the other hand, just reading books can be helpful but still not get you there. So reading the car manufacturer's guide or a driver's ed manual won't teach you everything you need to know about driving but it does a much better job at teaching you how the car works and what the rules of the road are.
Learning screenwriting isn't hard but it is made easier by having a driver who can help and answer questions. The difficulty with trying to judge a script by a movie is that they can be two different beasts. The movie is seldom based on the original script. The script is a starting point for the director and actors to re-imagine it. Many scripts you can buy are transcripts of the movie, not the original. Going back to the car analogy, you might have learned to drive your grandma's automatic (original script) but it's nothing like the manual 4 wheel pickup now in the drive (movie).
What do you think you need to learn? If everyone imitates 'successful' movies, they all become cookie cutter writers, which is often the case in Hollywood blockbusters. Unfortunately, Hollywood only hires insiders to write blockbusters. The indie film arena is much more open and looks for good storytellers. Understanding what makes a good story, how to develop characters, how to pace scenes and hold the viewers interest are skills that come from practice. There are good scripts made into bad movies, and vice versa.
If your goal is to learn screenwriting, study books that cover good techniques and then partner with directors and produced screenwriters to get feedback on your scripts. You'll learn more from feedback about improving your scripts than from other people's bad examples.
