I have to second the suggestion of the above Facebook group. The discussion board there is good for answering questions about the pros and cons of testing at the different DNA companies. It's also good for getting advice on how to interpret the testing results from different companies, and how to use DNA matches to find unknown relatives.
If you are concerned about Facebook and privacy issues - I have a very lean profile on Facebook, with a fake birthday and only the town that I currently live in listed, nothing else. I'm only on the site for the Jewish genealogy groups there.
Below is an article that the group moderator Jennifer Mendelsohn wrote about Ashkenazi Jews and DNA testing, and the issue of endogamy:
Lak/Lok/Liak/Lock and Kalon/Kolon in Zagare/Joniskis/Gruzdziai, Lithuania
Lak/Lok/Liak/Lock in Plunge/Telsiai in Lithuania
Rabinowitz in Papile, Lithuania and Riga, Latvia
Trisinsky/Trushinsky/Sturisky and Leybman in Dotnuva, Lithuania
Olitsky in Alytus, Suwalki, Poland/Lithuania
Gutman/Goodman in Czestochowa, Poland
Lavine/Lev/Lew in Trenton, New Jersey and Lida/Vilna gub., Belarus