Dr. Belfield has written quite a bit about ester-c and preventing or reducing symptoms of dysplasia and other joints problems. This is due to vitamin C's purported ability to affect tendons and ligaments surrounding the joint. I know several who believe it has worked well for them in their dogs as well.
One caution however is to understand that vitamin C is an acidifier and can dramatically alter the ph balance in the system. As a result, long term use should carry caution. For generally alkaline dogs, ester-c may actually be helpful by acidifying - but for dogs who are naturally quite acidic, vitamin C could spell problems, including bile vomiting, stomach upset and nausea including grass eating, and at its worst, calcium oxalate bladder stones and crystals (this is caused because high doses of vitamin C will cause rapid excretion of calcium in the urine). So just something to consider and evaluate before taking on long term use of vitamin c.