UPDATE. Well, the cruise is over. In some ways it was better than I expected, in other ways it was worse.
First, the (comparatively) good: There was a minimal number of kids on board and the ones that were there were pretty well behaved. I didn't see any children running around unsupervised causing a problem. Also, no drunken disturbances, fights, or outrageous rowdiness by anyone on board (although we never did go to the nighttime deck party or the night club). The Celebration being such a new ship, it was pretty nice and in pristine shape. I must say that this part was better than I had expected.
Now, the disappointment: food and food/drink service. Yuck. I didn't have very high expectations here, but I have to say that Carnival still managed to disappoint. The MDR was a joke. The food was like something out of a hospital cafeteria, and the service was abysmal. Our drink orders took forever and were usually wrong (or substituted do to inventory). Any cocktails ordered in the MDR or pool deck were horrible. This includes the saddest espresso martini I have ever seen. While the MDR staff was slow, at least they were pleasant. The same could not be said for virtually any other place we ate aside from the one night we did the steakhouse (which was actually very good and reasonably priced).
The biggest turnoff for us (other than the food) was the overall vibe. Carnival calls themself the "Fun" ship, but I have very different ideas of what constitutes "fun" than they do. The ship was very crowded, and we found the packed deck parties and constant commotion, obnoxious music, and whooping/hollering from the other passengers to be nothing short of annoying. We spent the majority of our time outside up in the Loft 19 area, which was decent but lacked a bar, conventional pool, or any sort of eating area.
I'm not going to disparage the other passengers or comment on demographics, etc., but I will say that there were noticeable differences between this crowd and what we are sued to on Celebrity. I won't say that it was a "problem"...but it was noticeable and awkward at times.
All in all, it was an acceptable time. We were there with friends, and that made a big difference. Had it just been the two of us, I would certainly have had a much worse time. Carnival is just not for us and I can't imagine that I would ever sail with them again.