I used it to trace a family tree and found that ancestors were in USA before 1750 or something (not to mention relatives currently in the US that I didn't know existed (some have several aliases looking about so, it could be worth ignoring those. Lol.), but I can only trace records in GB back to 1801 online. To research before that, I have to go out into the darkest backwoods and look in the original registers myself.
Generally, the records on Ancestry.com seemed accurate.