Many pre-meds take it as juniors. The paradox with many summer courses is that they're actually harder, but are not as highly regarded. I took biochem last summer, because I switched into a major that required biochem prior to taking classes senior year. It was brutal! It was scheduled for six weeks, but my first class was a Tuesday and my first exam (1/3 of the material) was the following Thursday, only nine days later. I had to memorize every step (structures, enzymes, catalysts) of glycolysis and gluconeogenesis, as well as enzyme kinematics. Ouch. It was worse than a full-time job...I was working full-time, and came home and studied three hours a night and all weekend just to get an 83% on the exam, which was an A. We had a 4 hour lecture the day of our final exam (3.5 hours later) which included that final lecture.
I think I studied far more for my summer biochem class than I would have had to study throughout a 14-15 week semester. We had less time in lecture, which meant I taught myself more of the material than I was comfortable doing. Still, others in my major consider me "lucky" for not dealing with the three demanding (and often incoherent) professors who teach it during the fall semester. I disagree, but I'm not going to argue the point.
Summer and winter sessions are better left for English and other degree requirements, which may not be easy, but require less absorption of facts than science classes require. For me, it's easier to read a book in two days and write an essay than it is to memorize every step of complicated pathways.
