I was on the Constellation in December, transatlantic from Southampton to Miami. I thought that all of the things you are inquiring about were good enough so that I booked an open reservation for a future Celebrity cruise, which I am going to take next month.
The cabins are similar to those on HAL and Princess; a tiny bit smaller than HAL, and the bed is more comfortable than those on Princess. I thought the food was generally very good, though I was surprised that after 8 or 9 days they started repeating the menus in the MDR. Not just some items, the exact same menu. Ice cream is better on HAL and pizza is better on Princess, at least in my opinion.
Evening entertainment was great, and unlike Princess the theater is large enough for the number of passengers aboard. During the day there were always more things interesting enough to do than I could get to, partly because I like trivia and they had about 4 games a day on the crossing. Luckily that meant I didn't mind missing some of them to go to a lecture or one of the sports games. They had a forensic investigator aboard who gave some very different talks that sounded from the description like they would be dull but turned out to be fun.
I've never gotten anything but excellent service on any cruise ship. I don't know if that's indicative of my having extremely good fortune or extremely low expectations!