I was a premedical member of AMSA. The only benefit I received which I found useful was a subscription to the New Physician magazine. It provides for some good bathroom reading.
I was only a member for a year though, since they raised the membership dues for premeds in 1997.
I recently joined and kicked myself because new members can receive a complimentary copy of the MSAR (purchased mine a month earlier). That right there is worth the cost of a year's membership. Don't know much about the other features yet although one should probably not expect anything too great.
Oops, I forgot to say that the MSAR is the Medical School Admission Requirements. It usually costs at least $25, so I guess the membership would be worth it just for that.
I joined about a year ago myself and was interested in their low-interest loans for med-students. I finally got the paperwork and unfortunately, the loans only seem to be for residents (travel/interview loans) and consolidation of your loans when your done. I may have just talked to the wrong person (or maybe they misunderstood what I was looking for) but it was a little disappointing.