Not sure I have much more constructive to add to this topic. But I'll weigh in regardless...
We started cruising X on our honeymoon cruise. Now 7 years and two kids later, having experienced HAL, Princess, and NCL, mainly due to price considerations, really have decided that X best matches our taste and cruising style. We're fortunately willing and able to pay more for the privilege, but have been thrilled to see the most recent Kids Sail Free promotion which has been a rarity for X in the past.
We've also taken our kids on previous cruises and had nothing but positive experiences, both with fellow cruises and the crew. I know we've all had bad experiences with "other people's kids", whether on a cruise or a plane or in the local grocery store. Frankly, I've found just as many rude non-parents as rude oblivious parents who don't seem to keep their kids under control. Rude, discourteous people everywhere you go, and it's admittedly hard not to let them ruin your vacation.
I'll also provide some reassurance: even with X's KSF promotion, it's still WAY more expensive than your Carnival or NCL party barges. I'll selfishly believe that the type of parents who prefer Celebrity also have the type of kids you won't mind having on your vacation. You'll be fine. And come say hi to my kids when you see them. We're on 3 cruises booked in the next 12 months. If they're out of line, you have every right to come tell us and we'll ensure they don't ruin your vacation. :)