You know the ones I'm talking about. "I put my grandfather's name in and a leaf popped up."
What concerns me about this is the "about the same" syndrome. How many times have we been researching John Jones and we find three. They are all born about the same time in nearly the same area....or not even the same area. They all married someone named Mary. Since they nearly share a birthday, all live in Virginia and all are married to Mary, they HAVE to be the same person. So that person imports and merges all the information into their tree. We all know that once something hits the 'net, it never goes away. That one assumption, that one innocent mistake has now the possibility to propagate over and over until it is believed as fact. You contact the people with the incorrect information and they refuse to correct it, even if you have proof to support you. People beginning to research their family have to remember to question everything and don't accept anything on face value.
On the other hand, Ancestry.com is great for finding records and making connections. I found emigration records of my adoptive 3g-grandparents when they came from Germany to the US in 1888. The search ability is a dream compared to the old days of sitting in front of a cranky (ha! get it?) microfilm machine and squinting. I found several cousins easily and was able to share information almost immediately.
So, is Ancestry.com a blessing or the Devil? I suppose it depends on if it's your ancestors that someone has attached incorrect information to.