I've used Ancestry.com with a little success tracking my mother's side of the family. The real problem is that her maiden name is so very common, as are many of the first names of my ancestors that it is entirely possible that you get a few different matches for (for example) a Robert Fairchild who was born to Samuel Fairchild in or around 1835. My family claims Scottish ancestry, and yet when I track what is available on Ancestry I get all the way back to Thomas Fairchild around 1610 somewhere in England - and it seems that most of the Fairchilds you can find all somehow go back to Thomas if you're using Ancestry and other sites that pull information from the same area.

But, those problems aside, it has been a useful tool to tracking down some other parts of my family after they came to America. For example a movie based on true evenets came out sometime ago that starred Matthew McConaughey that took place during the Civil War - well I found out that I am slightly related (through marriage) to the character Mr. McConaughey portrayed in that movie, and I found that out using Ancestry and verifying records that corraborated with my own. So not only did I get the excitement of having a movie made about my home county, but also that a movie was being made that told the story of one of my relatives.

The real problem with Ancestry.com is that most of their records come from the US, and the UK, so even with a paid membership it can still not be that useful if you're trying to track down family out of that area. For example, my father's side is Italian. With Ancestry (and even with the international membership) I can only trace my family back to around 1850 when my great-great-grandfather was born, and that comes from his ship manifest (given the date and the age he put down) and his Ellis Island record. But going further back than that is almost impossible if using Ancestry because they don't have as complete records for Italy as they do for the US, also many records would be kept at the local parish, and then lost if the parish ever shut down or mergered. BUT!!! I at least have somewhere to start looking should I ever get the money to hire a geneologist to track my Italian side.

I recommend using Ancestry, because the family tree is at least free to use, and you can upload your own records and pictures, so your children and their children can look back at these things as well (there is a privacy thing in place so other people can not view living people that aren't in their tree). And I recommend downloading your tree (which is also free) and uploading it to GEDCom (as another user did) and finding matches that way. If you do the DNA test you can download those results and upload them to GEDCom as well and find DNA matches all for free (save the initial cost of the DNA test).

Now, 23 and Me has an interesting thing where they will show you if you are related to famous people from history. My father did this and it showed a relation to Naploean Bonaparte, Charlemagne, and St. Luke. Keep in mind that this just shows possible relation, not necesarrily descent.

All of these can be great tools to use, but as with all tools, you have to put in the work to use them.