Ahhhh, summer classes. I took 2 (technically 4 if you count the 2 summer internships I did), and you are right...it's a lot of work. I know at my school they took the regular semester, and crammed it into 8 weeks; so classes met twice a week for 3 hours (ugh!). I took courses that I knew the professors in, and had previous classes with. I took Communication Law with one of my favorite professors, and I know what you're thinking "a law class? twice a week in 3 hour blocks?! Are you crazy?!"...my professor was so awesome that the 3 hours went by really quick. The amount of work can be intense, but if you plan your schedule right you can get your homework done, and have time to decompress and treat yourself to something that would make you happy.
As Jenna said, internships in the summer are usually for class credit, so that is another option for you to do in the summer. Plus, some internships pay, so then you can get a bit of extra pocket money. I enjoyed my internships in the summer, as I was able to adjust my schedule to get the real world experience I needed without worrying about skipping a class.
There's no definitive yes or no on this question; just go with your gut and what you think you can handle. If you put your mind to it and plan it out effectively, summer classes will fly by :-)