We are just back from a 4 day Century cruise, and overall it was a great cruise. It was my partner and two close friends, plus an amazing Cruise Critic group. We have done several short cruises on Majesty of the Seas and Navigator of the Seas, and while we like those as well (and will sail them again), this was a nicer experience.
THE SHIP - Other than the casino, the ship does not at all feel dated to me, and is very well maintained. The Martini Bar and Michael's Club are great spaces. Of course it does not have all the bells and whistles people have come to expect from the newer ships, but it is still lovely.
FELLOW GUESTS - since this was a short Caribbean cruise, I imagine the age range was a bit younger than the usual Celebrity cruise, but it is not a Royal Caribbean cruise and so there are fewer 20-somethings (or so it was on our cruise). But there was still a lively crowd in the Hemispheres lounge each night, and it is definitely not an "old" crowd (either in age or outlook). Since we don't have kids, can't tell you about the kids program, but others said they had good facilities on the ship.