You will find most people here are big cruise finds. We cover a variety of ages and economic situations. And if you look around you will find plenty of opinions.
Check out the home page of Cruise Mates. There are several articles there to help you plan a successful cruise.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014
Star Clippers: Royal Clipper 2015
its really difficult to give a general overview, because no two ships even in the same line are the same, they each are unique in their own way. Your cruise choices depend so much on what you want to do, what you personally want out of your cruise experience, if you could give us some more details we could give you some specific advise
If you go to the home page of CruiseMates, and click on Princess Crusies, it will give you an overview of the cruise line. I think it's a good place to start, and you might find some of your answers. Happy Cruise.