Freddie, my sprollie is 19 and has been on metacam. I have noticed a marked deterioration in back leg strength and the start of (manageable) continence issues. Its been suggested that metacam can reduce muscle tone more quickly than would otherwise occur. He's actually been off it for a couple of months and has coped well in the summer.
Denes Greenleaf tablets have been recommended and seem to have good reviews online.
At 19 its impossible to judge whether metacam didn't agree with him, but the strength issue was marked from when he was put on them. He's had an ace summer, with his own mattress outside in the sun to lay out. I have underfloor heating. So I don't plan to restart the metacam if I can find something else. My vet is great... but doesn't really listens, he lets me speak and then gives his patter and doesn't really answer my VERY specific questions.