It is self reported, self selected and is more focused on numbers than the actual application process.
The problem with it being self reported? The gpa and/or mcat numbers may be fudged to make someone look good. People can make fake profiles.
Self selection means that perhaps people with lesser numbers don't enter their data into mdapplicants. This may be particularly true with SDN because many of us have links to our mdapplicants profiles. Maybe you don't want people to know your mcat so you never fill out a profile. As a result, your acceptance or rejection from medical school is never recorded on this site.
Finally, the actual process also takes into account our interview skills and essay writing capability. Someone could have a great gpa and mcats but send in an error riddled personal statement, interview poorly and/or have awful letters of recommendation. Than you see his/her mdapplicants profile and are sitting their scratching your head wondering why they didn't get into medical school.
All of that being said, I have gone there many of times to compare and contrast. My gut feeling is that the people with scores around average and lower are more reliable than those with upper range scores. Plus, it is really the only resource we have!