I have had my DNA tested; I did it with the general idea of helping science to make advances, but also with a hope that it might somehow be of use in finding ancestors.
My testing was with FamilyTreeDNA.
DNA testing is not inexpensive, and my opinion is that it is unlikely that it will help you find any ancestors until the databases are much larger (but there's always a chance).
The "Family Finder" type of test is probably the one most likely to bring you into contact with another person who is looking for ancestors, and who shares some of your own ancestors. The typical problem is that your common ancestor is too many generations into the past.