Just returned from a b2b cruise on the Island Princess. After cruising Celebrity and Princes for several years, I can say that service was good on Celebrity but the service on Princess was definitely a step above. I have seen the raves about Celebrity's passenger to crew ratios in their advertising promos, but IMHO, Celebrity still does not surpass the service and the friendliness of the crew on Princess.
There are also many other things to consider when selecting a cruise line to spend your vacation dollars with. The service level is certainly one of the things to consider. Other things that IMHO are even more important are the true character of the management of a cruise line and how they handle problems when they do come up, the way that a company maintains their ships and the reliability of a cruise lines ships. Even the past passenger programs offered to show a cruise lines appreciation to past passengers is also important. On these last issues for consideration,
my experience has been that Princess wins hands down when compared to Celebrity.