We've been in an 11 or 12 several times. Of course, you don't have a romantic breakfast on the verandah.....but we spend very little awake time in our stateroom, so it is just fine. I did have a cat 6 once. That was in July (remind me to never cruise in July again.) It was 106 degrees the day we left Florida, and the verandah railing was so hot that you couldn't touch it. The only time it was worthwhile was to see the view of Castaway when we docked.
I've convinced my family that 2 cruises in a gty 12 is better than one cruise in a 6!