I took Kaplan 2 years ago, and the best I could muster was a 25P this past August. I did OK on the PS and BS, but horribly on the verbal. Kaplan obviously didn't help me in the verbal department. I was alright with the PS material, but spent half the time lost with the biology. It would've helped me to have taken some relevant upper division classes before taking the prep course.
The teachers were insightful (they were all students at OHSU), but the commute was a headache (40min each way, $5 for parking).
Basically, if you have the $$$ and need structured curriculum, then I would recommend it. But, you need to take advantage of their study materials, or you will gain nothing. If you are motivated to study on your own (or with friends), then save yourself some money and buy some books (examkrackers, kaplan, etc.) and practice tests.
Also, my classes were on Saturday and Sunday, which gave me classes 7 days/week. If you decide to opt for a test prep course, schedule your classes carefully.