(Disclaimer: I have been a subscriber to Ancestry.com for several years, and as a paying customer, feel that I can give an honest critique of the service.)
First, I believe that their charges are too high for a lot of researchers. I do not know what a membership permitting access to everything on the website would cost but, I have access to just a few records, including the census records, and pay over $200 a year. A SWAG is that access to everything would cost over $1000 a year. I have no idea what a fair price would be, but I suspect that it would be something less than a grand!
I don't know about the rest of you, but I find the family trees on Ancestry mostly useless. They all appear to be duplicates of each other, with no sources given. If a source is given, it is typically an FTW or GEDCOM file that someone else has already posted there. If a legitimate source is cited, it is often incorrect (I have seen the Social Security Death Index used as source for death date of someone who died in the 1700's!). I have often wondered why some researchers are so opposed to providing sources.
One thing I like about Ancestry is their census collection. For the most part, the indices to the census files are good, but I caution users to not use exact surname spellings. I most often use the soundex for searching, then narrow down large returns by including additional search parameters.
One good feature is the inclusion of other family members in some indices. For example, if I am searching for John Smith, and know that he had a wife named Marion, and a son named Frank, I can quickly look down the index search return for John Smiths, and look for the one with family members with those names.
I do not depend entirely on the indices prepared by Ancestry. Sometimes I know with certainty that a family is in a specific state and county, but the index cannot find them. Perhaps the person extracting information for the index just skipped them, or otherwise did not see them. It is in instances like this when I just have to page through the entire county, or township to find the family I am looking for. Ancestry does make provisions for reporting bad census information, but they don't make it easy to use. I am not sure what I do incorrectly, but the submission has failed each time I have attempted to send them corrected information.
One last thing: Their search engine, especially for old newspaper articles needs improvement (or, perhaps I'm using it incorrectly). But, when I enter a name like Francis Scott Key (extreme example), and get 50 hits for Francis Bacon, Scott Tissue, and Locksmiths, something is wrong.