I took a couple of literature classes in the summer to catch up on my hours. Otherwise i wouldn't have been able to graduate on time.<br />
I thought it was a good experience overall. Taking a few during the summer lightened my work load during normal school hours so I felt better paced - and I didn't have to move all my stuff out of the campus apartments. <br />
It was more work, though. It was every day, instead of every other day, and my classes were online so it was constant reading and writing.
Last summer I took an intro to bio class(8weeks) it was approx 4 hours Mon-Thurs. I probably could have handled one more class maybe but it was intense. Anything over 8 would be awful hard. My $.02
I took summer spanish, it was 6 weeks and 6 credits, pretty brutal. Like 6 hours a day. But I really didnt want to take my foreign language and it at least knocked it out quickly.