I may be one of the few here who have cruised with Celebrity both before and after it's acquisition by RCCL. Under original owner Chandris (hence the "X" logo in a Greek language meaning) the line was truly upscale. Wonderful food and dining room service with staff provided by Apollo Caterers. It was almost a 1:1 ratio between staff and guests. Extra charge venues were few. After RCCL bought Celebrity many of us saw the line lose some of it's luster. Please don't take this as a slam at RCCL, which by the way is not RCI but rather the corporation which owns both lines (plus Azamara Club Cruises). However, RCCL changed things at Celebrity and expanded the line. Celebrity more than doubled in size and as is often the case, when you get bigger you lose quality. Of course, this isn't the only reason but it is a factor.
All that said, I still like Celebrity a great deal. I will also say it is one cruise line that can really be a sharp contrast for some passengers. It is easier to have a love/hate view of Celebrity than it is with many of the big cruise lines.