We were in a CC on Century a couple of years ago. This was one of the cabins that was originally an oceanview and had a balcony tacked onto it during a refit. So, the cabins were not larger than the non-CC veranda cabins one deck below.
Yet, we did notice a difference on the M-class ships. In this case, it was our traveling companions in Concierge Class while we were in a standard balcony cabin. Their cabin was slightly larger and had a slightly longer sofa.
Our service was fine when we had the CC on Century, although I seem to remember them running out of something -- a key card wallet, I believe. No biggie.