I just had a conversation with Neiko's ortho vet on this subject. He said that human glucosamine has a different pH than that made for dogs, so the bioavailability of the human brands is much lower (a lot of it just passes through). He also said that he has a lot of clients that are human drs and get the human glucosamine for very cheap. There's no harm in that, but it won't be as effect as something like cosequin. Personally, I've only used brands made for dogs, so I don't have a comparison.
I used Synovi G3 for about 2 years with Maddy after her elbow dysplasia surgery. I honestly feel that the supplements I give her now are working better than when she was just on the Synovi. She takes a human form of G, C, MSM, and Fish Oil, and then she also gets the doggy Glycoflex. Since she has been on this combo, even if I run her in the yard really hard, she doesn't get as stiff or gimpy after resting as she used to.