I think those websites are fun to try, just to see what happens. However, at the end of the day we'll end up changing our minds so many times in med school, that who's really to know what field we will be in.
I just saw my former MCAT teacher that I'm always talking about. Anyhow, last summer when he was starting, he was all about family practice. Now that he's done some surgical research and observation, he's sort of liking that too. Similarly, I was at a premed forum one time and this fourth year said that she started with hopes of pediatrics, only to change her mind and go the OB/GYN route after gross anatomy led her to a fascination with the reproductive system. So who knows what will happen. I've taken the UVA survey that was linked above, but I'm going to see what the other one will give me as results just for fun.