that's a great question.....
well first of all....testable material is almost wholly from the notes. therefore if you don't want to go to class and feel that you are comfortable studying on your own, than that works. most people attend most of the lectures. mostly because it's just like reading over the notes the first time. there are some classes that it's a good idea to attend. for example, physio ( depending on the lecture), neuroantamoy, histology, sometimes gross anatomy, etc.....
it all depends on the individual. as far as performance vs. class attendance. there is pretty much no correalation. when a point is brought up during lecture that isn't contained in the notes, the info is shared with the class. and this only occurs very rarely.
and yes, if I am not understanding something thoroughly then i will attend lecture. ( but in general, med school material isn't difficult, it's just managing the large volume of material and the rigorous schedule).