I loved taking courses in the summer, especially in University, because instead of 5 classes demanding my attention, I only ever did one or two during the summer, which meant that my grades for sumer courses were WAY better because I could devote more attention to them.
Basically: every course that I had a problem motivating myself to go to (2nd year stats requirement, I'm looking at you) I ended up doing in the summer because I knew if I tried to do them with a full course load, I would probably do poorly because I'd skip all the work for that class in favor of the more interesting classes. I'd much rather write 15 pages on an experimental social model or David Hume (yeah, that's right: I have an Ethics Philosophy minor) than work on a spreadsheet.