I don't have experience with cats, but in dogs, these types of supplements are almost miracle "drugs" in dogs with joint issues.
The biggest issue with this type of treatment is managing the animals activity. In a lot of case -- at least with canines -- when positive responses are shown, the owners tend to increase activity since the animals seems to have a good reaction to the supps. The key is managing activity in a conservatory manor to leverage quality of life.
Again, I don't know how cats will react or if this is even a concern for them, but it's worth mentioning.