I've tried it out and you don't have to have a tree on the site, but you do have to have registered. I found that if I compare a photo of me when young, then I get 45-50% matches with my parents at a young age, but if I compare photos of me when young with older versions of my parents the match rate goes down. Interestingly, my younger brother and I were 72% alike, not surprising really. My daughter was 50% but my sons only around mid 30% - I guess that means they are more like their dad than me. (They've all got the same father).
What it did do was show me that I am more than 30% similar to the man I think is my g.grandfather whose origins I cannot find, but in general I am more like my mother's relations/ancestors than my dad's.
Like Martin, I found the older I got, the less I looked like my young self.
