Is concierge class worth the extra $? Yes and no. IMO, much depends on the ship, where you are sailing, and the importance you place on Captain's Club cruise credits.
We had a CC cabin on Century a couple of months ago. Our cabin was exactly like the non-CC balcony cabins one deck below us -- same size and layout -- and same size as the oceanview cabins (aside from layout and that the OVs have no balconies). We probably had different bath products, but I don't recall.
We visited an OV cabin, and the main differences I found were that the CC cabins had towels and robes that were more plush, we had a fresh flower in the bathroom, and had more items on the breakfast room service menu. We did not need other services on this cruise, such as priority tendering, as we docked in every port. Priority disembarkation was nice, yet we didn't need the priority embarkation line as the Captain's Club Line was shorter. (Of course, this last one varies from cruise to cruise.)
We did think this a pretty economical way of getting an extra cruise credit toward our Captain's Club membership. It was enough to push us up to the next level.
A year ago, we had a standard balcony cabin (not CC) on Infinity, and our traveling companions had a CC. Their cabin was more spacious than ours.
So -- all that said -- I'd consider it on the M-class ships (Millennium, Infinity, Summitt, and Constellation) mainly due to the extra cabin space, or on a ship that tenders in several ports. When we return to Century (and notice I said "when," not "if"), and if there are not tendering ports, I'd save the $. JHMO.
