Here is the deal:
They are ALL corrupt. I think all of them are facing conflict of interest lawsuits with auction houses.
I focus mainly on Barack Obama's autographs when searching the web. I usually look at them on eBay and have seen so many Obama autographs that I can tell you whether one is authentic or not with a simple glance. Here is the break down:
I have NEVER seen PSA DNA authenticate a forged Barack Obama autograph. All authenticated ones have been authentic.
I ALWAYS see forged Barack Obama autographs authenticated by GAI, in fact, I have only seen one authentic one authenticated by them.
I have seen a mix authenticated by JSA. They have authenticated both authentic Obama autographs and forged Obama autographs.
I would not recommend authentication by any company. In a hundred years, these companies will not be around due to corruption charges. Therefore, when an appraiser or buyer sees one with the company sticker, they may have second thoughts with purchasing or valuing one.
Please read
http://www.autographalert.com 's blog to read about all of the corruption in the third party authenticating field. They do a great job in blowing the whistle against these companies.
(If you MUST have one authenticated, go with PSA DNA)