I have cut and am pasting an answer as we get this question a lot.
Ancestry.Com's real value is the original source records it has online. They have all the U.S.. censuses through 1930. The 1940 and later are not available to the public yet. They have U.K. censuses through 1901. They have military, land, immigration and other records. They have transcribed the records but you can view the original images. Now, there are errors in their transcriptions, particularly censuses, but when you view the originals you can have pity on the transcribers.
Not all records are online but the ones they have will save you time and money traveling all over the country to courthouses, libraries etc to obtain them.
A word of warning: Be very very cautious about information in family trees on their website or ANY website, free or fee. They are subscriber submitted and mostly not documented or poorly documented. You might see different info on the same people from different subscribers. Then you will see the absolute same info on the same people from different subscribers but that is no proof at all it is correct. A lot of people copy without verifying. Errors in family history have multiplied because of online family trees. The info can be useful as clues only as too where to get the documentation. Documentation is the meat of genealogy. However, since you have been involved in genealogy already you probably already know this.
I have been giving this warning for 2 years on this board. I recently had my own experience. I found out that me, my sister and my brother-in-law are all dead. No date of death given but we all died in Newton, Sussex County, New Jersey.
The only time my sister and I have ever been in New Jersey is when our family drove through there going to and from New York in 1956. So we have been dead 52 years. I started checking further and found family on both sides that married and died in Newton, Sussex County, New Jersey. Since my ancestry is primarily southern American colonial (there are a few exceptions) I was very surprised.
I found this tree on Ancestry.Com. If this tree had been submitted to any of the other genealogy websites, Rootsweb, Genealogy.Com, FamilySearch.org etc. it would have been accepted. You can make up an entirely fictitious family tree and it would be accepted. If you disagree with anything another subscriber has on your family, the owners of the websites will tell you that is between you and the other subscriber.
If you decide to put your family tree in their Public Member Tree or their Personal Member Tree their system will give you hints to records they have in their system that appears to match people in your tree. Just make sure it is the same person. Frequently in times past, several people in the same family will have the same name livinig within a reasonable distance from each other. One of my great great grandfathers had a brother, son, grandson and nephew named Zachariah Berry Jackson and there might have been others. So you really have to be on your toes.
Also they will give you hints to people that their system shows in other people's family trees. Watch It! Don't go adding spouses and children to someone in your tree just because someone else has them. Good genealogy means a verifiable family tree not seeing how many names you can add to your family's data base. The family tree that had all the wrong information on my family has almost 150,000 names. I believe it is impossible for anyone to come up with verifiable information on that many people unless they have been working on a tree constantly for 40-50 years. They would have had to spend a fortune. Unfortunately it appears a lot of people think getting as many names as possible in their family history data base is what is important. Documented information is what is important and the more documentation you can have on any one person the better.
A wonderful source is a Family History Center at a Latter Day Saints(Mormon)Church. They have records on people all over the world, not just Mormons. In Salt Lake City, they have the world's largest genealogical collection. Their FHCs can order microfilm for you to view at a nominal fee.
I have never had them to try and convert me or send their missionaries by to ring my doorbell. I haven't heard of them doing that to anyone else that has used their resources. Actually, I understand a lot of their volunteers aren't Mormon.
Just call the nearest Mormon Church or visit their free website, FamilySearch.org, to get their hours for the general public.
However, you should try and get as much info from living family as possible, Find out if any has any old family bibles. Ask to see and make copies of birth, marriage and death certificates, and depending on the religious faith, baptismal, first communion, confirmation and marriage certificates can also yield information.
Interview your elderly. Tape them if they will let you. They might be confused on some things but what might seem to be insignificant story telling you wouldn't write down might turn out to be very significant.