Never used it in sheep but I had a collie that was on it for years and it worked on him,
Similar here. Pet collie cross with terribIe hips and arthritis in later life. I didn't give it to the dog every day, but would use it if I knew he would need it - the day before, the day of and the day after a long hike, for instance. It definitely helped him.
On the vet's suggestion I also gave him glucosamine every day and did think that this helped too.
One problem I had with the Rimadyl was the cat stole some of the tablets one night when my back was turned - they make them 'appetising' so the dog will eat them, and clearly this also made them appealing to my greedy puss! That dose was very dangerous for a cat, though, so it was a late night speed across the moors to the vet and a very expensive overnight stay and stomach pump