I just finished taking a year of general chemistry over the summer. The class was taught on the quarter system, with three classes of five credits each. The first two were four weeks, the third quarter was only three weeks. Here's my opinions:
The material is all the same. Nothing is cut out. Taking one class over the summer is like taking three during the regular year. It might not be easy, but it's far from an impossible workload. You won't be able to take two difficult classes or to work full time and take one class, but a schedule of physics and an easy class or a part time job is possible.
The class is fast paced, and you will need to be very self-motivated to stay on top of things. If you can't make yourself study every day and attend almost every lecture, you will fall behind, and there is no time to catch up. My classes had a test every week. On the bright side, you won't ever have multiple tests in one week like you will in the regular year, and studying for exams, especially the final, is easy since the material is so fresh. There's only one class to worry about, so it's easier to keep track of what work you have to do. I feel that I got a good education, and I understand general chemistry at least as well as if I had taken the class during the regular year.
Classes are smaller, and the students are generally better. Everyone wants to learn, since the people who go to college to smoke dope and play video games generally don't take summer classes.
In general, I would recommend summer classes, as long as you can stay motivated enough to keep on top of the work. The only exception would be if you were in a situation in which you absolutely, positively had to get an A. There isn't much flexibility when it comes to taking exams, so illness or a death in the family at exactly the wrong time could spoil a good grade. The combination of a staph infection, a tough chapter on redox reactions, preparing to move to Alaska and the final exam all in the same week very nearly knocked my grade in the last quarter down a letter.