I second the vet before giving any supplements due to a heart murmur, you just never know.
As for Cosequin, I give my older dog (12) Cosequin DS (half a tablet a day). He isn't arthritic, however, he was creeky and slow. Read that as he'd get up and be stiff for a bit (so am I, maybe I should take some of it too). After giving him Cosequin, for the most part, it seems he's moving a lot more and isnt' stiff. There are times, like when he's been laying on the hard ground for hours, that he wakes up stiff, but hopefully a new bed will help with that.
For myself, I've seen it do great thigns for Miko. However, like I said, he's not arthritic though. The Vet may even give you something more potent if it is arthritis. I know my co-worker got Dasequin (vet version of Cosequin) for her Lab and she said it helped a lot to alleviate the pain/stiffness.