I think Ancestry.com is a remarkable source of information, but as with all research, you must be wary of what you accept to be true! Access to public records in an indexed format that can be searched rather than browsed, is outstanding, and they are always adding more. I am suspicious of local histories and family histories (particularly those written about 1900), but they can be a great source of names and locations that should then be verified independently.

The public trees are also helpful if you check to see if the data is documented or verify the information yourself. People copy from other trees indiscriminately, then other people copy the same mistakes over and over, again. My favorite was one of my Scottish lines back in the 1300's where someone repeated the information, including the dates) for about four generations in the middle of the line (i.e., 1,2,3,4,5,6,7,3,4,5,6,7,8,9). I found this error copied to about twenty trees, and this was for a fairly well documented, noble family, for whom it was easy to verify information.