I just received a wonderful link in my inbox. It's a sorely needed article on Ancestry, explaining in general the difference between original and derivative sources, as well as primary and secondary information. I have heard too many times something like "I traced my family tree back to the 1500s," while in the fact the person has taken all or most of their information from a family tree on Ancestry, with little or no source information.
I have used public member trees extensively on Ancestry, with great success - but, they are just a clue, to point you towards (possible) evidence of your ancestry. As the article says, "tying into a database doesn't suffice as proof."