I have a friend who went to Harvard summer school. I think she applied but like timetravelerswife said, it's not particularly selective. If you have the money, you can go. Her classes weren't super had; I think it's more supposed to be for fun and fullfillment.
Colleges will not discount your experience, but they will know you paid to go and it won't look as impressive if you organized a volunteer project or got a job. It's not going to get you super kudo points on your application. I'd go because you want the experience and want to have fun, not because you can put it on your resume.