Has Celebrity slipped? Some people writing here think so. We've sailed many times with Celebrity in the last 10 years, the latest in June 2013 around the British Isles. My view is that, on Infinity at least, Celebrity is still a well-run operation, and if it has slipped at all, it is down from excellent to very good. My wife is a stickler for cleanliness, e.g., and the ship certainly passed muster with her.
My main complaint is that, like many cruise lines, I believe, Celebrity has introduced more classes aboard ship, the better to squeeze more money from hapless passengers. We were talked into booking on Aqua class, e.g., which, for considerably more money offered us a cabin with such things as thicker towels and a bigger shower head. Gimme a break. Not worth it. Those traveling in ordinary indoor or outdoor cabins are disadvantaged by these additional classes above them, because these classes are, I believe, given priority on embarkation and disembarkation, thus inconveniencing others. It's a gimmick, not worth it, and just bothersome to us "classless" Americans.
This is subjective, but my wife and I felt the breakfast buffet was not up to what we experienced on Celebrity years ago. But it's marginal.