Not accurate because your evaluation of yourself is subject to bias. It is easy to decipher what the questions in these tests are 'really' asking. A lot of them are not very well disguised. With each question you answer, you have a rough idea of which specialties are being crossed off and which specialties are kept by the system. I think most people just trick themselves into thinking they are answering the question objectively. In reality, they are just picking the answers they know that will most likely lead to the specialties they are interested in, which can be influenced by things such as the perceived pros and cons they assume about each specialty.
If what I am stating above is true, then might as well not take the test at all. Right? You already know what you are interested in. Maybe it is for people who want reassurance? Or just for fun? That is just my take on it...