How accurate are the genetic genealogy tests?

I looked at this one site, www.dnaancestryproject.com and im wondering, is this worth doing? How accurate can this really be and how much information can they provide me? Any ideas or suggestions will be appreciated.

DNA testing can tell you in which areas of the world your ancestors have probably lived over periods of thousands of years.
Your DNA results can also be used to connect you, if you are really lucky, to other people who may have different surnames to you, but who share the same DNA proving that you likely share a common ancestor. It can also prove a more recent connection to someone of the same, or even a different, surname in the event that you do not have a paper trail to prove that link.
The results of a DNA test are generally extremely accurate,
it is a new science which is evolving all the time, but for the purpose of genealogy it is really quite adequate.
I have used Family Tree DNA and can thoroughly recommend them, they are also the largest and most experienced company in the field, their site will give you all the information you might need to make a decision.
Good luck !
http://www.familytreedna.com

What they can tell you is what regions of the world you have genetic markers in common with; meaning that those are the regions that your ancient ancestors most likely came from. What they do not and cannot tell you is who those ancestors specifically were, individually. They do not have DNA of specific people to be able to tell you that your 20th great-grandfather was King so-and-so, or anything like that.

I'm not certain on the exact information that they provide to you (i'm sure you can find that out from their website), but as for accuracy - 99.9%.

Considering DNA tests are used as positive proof in court cases and in paternty tests confirming a biological father, I'd say thats pretty darn accurate.