CuriousAdcom,
I was just accepted to medical school, so I can't speak to sdn for medical students, but as a premed, it has been absolutely indispensable.
I started out as a nontrad two years out of school with no access to premed advising, nor to any doctors anywhere in my family or friend circle. In fact, when I was interested in applying when I was a student, I was told point blank by a premed advisor (years ago) that I didn't have a chance of getting in. Yet, my childhood dream just wouldn't go away, so I was going to try to make it work.
The only things I knew were that I needed more volunteer experience and that I had to take the MCAT, which had information from some classes I hadn't seen since 1999. I didn't know when, where, or how to apply, when, where, or how to study for and take the MCAT, which schools were in my home state (Texas), or any kind of timeline for applications.
Pouring my heart out as a hopeless newbie and trying to get over the feeling that I was crazy to leave a worthwhile profession, sdn provided the support I needed and pointed me in the right direction to get information. I gave a detailed account of my experience in the "What are my chances" subforum and asked for a reality check. The adcoms counseled me to add shadowing experience to the experience I had from 14 years ago; one even provided his phone number over private message so I could grill him with questions - he spent a full half hour with me until I ran out of questions. I was able to get recommendations on the best books to buy for MCAT prep and given the site to register. Someone set up a thread specifically for those taking the MCAT the same day as me, and we challenged each other with study strategies, practice test scores, daily quiz questions and explanations to things others found confusing.
Later on, school specific threads provided secondary application essays for prewriting, interview dates as members were invited for interviews, and pointers/reactions for those who had upcoming interviews. We consoled each other when rejections came and cheered each other's acceptances. More general threads served as hubs for those of us who shared common interests -- a Texas specific thread, threads with links to interesting articles, threads for those of us who were panicking, a whole forum specifically for nontrads.
In short, sdn is a community and after I found and became active on this site, I never again felt alone in the application process, despite knowing no one else in my personal life who has gone through it.