Summer school for the premedical student has both its advantages and disadvantages. I would argue that you must be very strategic in making this decision. One very real reason to enroll would be because you failed a required premedical course during the regular academic year or were very close to earning a "C." If this is the case summer courses are essentially required.
The benefits are that you are not delayed in completing all your required premedical coursework or having to reschedule the MCAT. Also, you do not have to take multiple science courses at once during the academic year to compensate for poor performance.
Another advantage is that your focus is solely on the class which you are enrolled. As a student you do not have to be concerned with extracurricular activities, doing research, or volunteering, your priority is to excel in the classroom.
Lastly, you have the option of choosing where you will take the summer school course. It can be done at your home institution or another school of your choosing. It should be noted that many medical schools will frown upon taking a premedical course at an institution which is not comparable in academics to your home institution. Additionally, if not at the home institution check with your advisors and the registrar's office to ensure the credits will transfer.
Summer school is for the student who can put all distractions aside and focus on learning in a rapid-fire environment, handle exams on a weekly and/or biweekly basis, and comprehend new facts continuously. It gives an insight into the life of medical school, yet still not at the difficulty level.
Generally, your regular financial aid package does not cover summer school tuition or the other expenses incurred (room & board, books, lab fees, etc.). Therefore, be prepared to pay the full balance at the beginning of the course and look for additional financing (relatives, scholarship, or loans).
With the rising cost of tuition, students will turn to community colleges for their premedical courses but it is important to take the class at a school on par with your home institution. Medical schools do not look kindly on applicants who take a majority of their required courses at inferior institutions.
Finally, you will be attempting to complete a full academic year in 6-10 weeks, which means your time will be severely limited. You should not plan on working, volunteering, or completing research. The goal is to study, study, study and pass.
I completed biology and organic chemistry at Harvard University Summer School and it was a very enlightening time. You had very smart people from across the country working together on: late night library sessions, rigorous review sessions, hectic planning for upcoming laboratory reports, etc. Since everyone came from different undergrad colleges we found that competition was nonexistent.
If the pros outweigh the cons for you then go ahead and enroll. Obviously, I was at an institution other than my own and it was great to get away from the midwest and experience more of the east coast. You are going to study a lot and its not because the material is harder, rather because there is more of it to be learned.
I want to mention that there will be times when the books can close and you can enjoy yourself, generally the night after an exam, but be prepared to be back in the classroom the next day. It's important to continue to do the things which you enjoy and for me that entailed staying true to my fitness routine, otherwise one can go insane if they only had their books for the summer.