Most of the time I spend on Ancestry and online is accumulating scans of the information they already had. When Mom and Grandmother were doing their research, they could only look at census indices - I can now get the scans of the original pages. I'm filling out the blank areas of family history and learning more details about my ancestors.
I have managed to take some lines a lot farther back just from online research. I'm lucky that many of my early families were Quakers, so the records are fabulous.
I got back into genealogy while I couldn't stand or walk much so online research was pretty much my only option. While a lot of the same info is available free from other sources, Ancestry has better search engines.
I seldom use other people's trees or the postings in Rootsweb. I go directly to original sources such as census, or verified secondary sources, such as transcriptions of town records, local histories, and some published family histories.
Our local library system is small and does not have any genealogical services so if I need anything from behind a pay wall, I have to pay. Considering the volumes of material I have downloaded from Ancestry, the library would be really fed up with me monopolizing their computers, anyway.
For outside of Ancestry, I use a lot of state archive material that is online and fee. South Carolina is wonderful - they have a lot of documents, wills and deeds, that help a lot. I verified one of Mom's ancestor's wife's maiden name, her father's, mother's and grandparents' names and proved that the wife of another ancestor was a sister through a series of wills, estate papers, and deeds.
Virginia has a lot, but their indexing is not as easy to use as SC. North Carolina is county by county, but I'm lucky that Chatham County is excellent since one branch went through there. Pennsylvania's archives online has tons, but their indexing is archaic - you have to use the original indices to find where to go for deeds, for instance. But I did manage to find the deeds for the land my ancestors bought directly from William Penn!
I've scrounged every bit of data I could get from the USGenWeb Archives for the places where my ancestors lived. I've looked at DNA family projects for clues even though I have not yet had my DNA sampled - I did find a clue that gave me another family branch back to their arrival in the Americas, even though that was not the DNA project I was looking at!
I've checked FamilySearch.com - the LDS site - and had limited success with them. FreeBMD.org.uk gave me some help on my United Kingdom branches.
So I am not just relying on Ancestry - It's just the easiest way to get the scans of the papers I want. Where it was great was for researching my BIL's family. His father's people came from Hungary (I'm not sure what country it is in at the moment). On Ancestry I found the passenger list for the ship where the immigrants came over and that named the villages they were from. Ancestry had just added records from those villages so I was able to give him much more detail than he had ever known!