My girlfriend and I have never cruised Carnival, but we look forward to it one day. Our personal feeling... Norwegian cruises are a last resort.
We have cruised with them 3 times, mostly because of their prices and ease of location, but there is absolutely no WOW factors to a Norwegian Cruise.
The meals are ok, staterooms adequate, staff are great. Shows, ok; Freestyle is nice. We always say, there is nothing wrong with a Norwegian Cruise, but there is no WOW factor.
Our last cruise in October on NCL was nice, but when the weather turned cloudy and cool and everyone was stuck inside the ship; the cruise director staff seemed ill prepared, very few things scheduled, and you felt on your own to entertain yourself.
This is just two girls opinion; personally, we are looking into Carnival for our next cruise, hopefully we can make it work.