I've been using Ancestry for a couple of months now. I have found numerous errors in the data, which I've flagged with them. They ask for your e-mail address when flagging errors but I haven't received a single response.
I can find records on "free" sites but not on Ancestry. I've queried this with them and they say that they don't have "all" records.
When you tick the "exact" box, you still get a mixture of records, so what's the point in that!
On a positive note, I have found a lot of information and the public family trees have been really useful, but you can't believe everything you see. I've found lots of inaccurate information!