These things are fun (and I've done this exact test), but please remember that nothing is 100% accurate and don't take the results to heart. It's a fun way to see if your guesses might be right, but there's also a chance that the results can be skewed (or that your dog might look like something but test as something else).
I tested my little brown dog a few months ago and she came back mutt/whippet mixed with GSD/mastiff. But she has the looks, instincts, drive, and personality (and has been registered through United Kennel Club) as a purebred Kelpie. This is the picture I sent in to UKC to get her registered:
[MOD: Changed so we could see pic!]
She's got the ears of the whippet (but also Kelpie ears that just didn't stand up all the way) and the herding instincts of a GSD (though Kelpies are also herding dogs), but other than that I found the test kind of a joke. Mastiff? Really? She weighs 38 pounds and is fairly thin boned and has zero mastiff tendencies. The shelter I rescued her from had her listed as a Rottweiler/GSD mix - and I told everybody that that idea was kind of a joke. I DO have a Rottweiler/GSD mix and she looks nothing like this little brown dog here. Totally different personality, too!
My bosses also tested all four of their dogs with the same test and had some pretty whack results as well. Since all four of their pups are total mutts, it was interesting to see what they came up as, but, again, nobody took the results to heart.
Wisdom Panel includes a little section on each determined breed and tells you to use it as a "training tool" when doing training or activities with your dog - that part is the biggest joke of all and please don't think that just because your dog has part GSD you have to train it like one - each dog, especially mixed breeds, are totally individuals and learn in their own way. Find a reputable trainer that you enjoy working with and they will likely help you work with your dog on an individual basis.
But if you've got a Heinz 57 and just looking to do something kind of fun for your pet (it really is kind of exciting waiting for the results and reading them when they first get published), buy one. I don't regret doing it, and maybe I'll do a re-test in the future to see if I get the same results.