I met a couple on the Norwegian Star last March that have been on over 70 cruises and another gentleman from New Orleans that had been on over 50. I believe the vast majority of both their cruises were on the Norwegian line.
I like Norwegian (this was my first cruise with them) but I don't pick my cruises by line. I pick by destination, date and price, in that order. Now, I do miss the whole loyalty program perks, but that is the price I pay.
I have only been on three cruises (Royal Caribbean, Carnival and Norwegian). I am honing in on a Princess cruise for my next trip (10-day out of San Francisco is looking tasty) and I don't feel like a dirty little cheater