I record all of my lectures and listen to them while I'm studying at home, in the car, walking the dog, etc. My classes are typically four hours long, so I record the lectures in chunks, typically by chapter. As the professor pulls up the powerpoint for the next chapter, I start a new recording and title it with the class and chapter. It makes it so much easier to find what I'm looking for while I'm studying. I haven't had a professor say no yet, but I'd definitely make sure to ask permission before busting out the recorder.