Always enjoy the atmosphere in Carnival's casinos, and their comp program has been rewarding. My first 7-night cruise with them, I played blackjack at $25/avg for no more than a total of 12 hours, and I received a comp'd balcony room on a future sailing in the mail shortly after.
Their blackjack table rules are a bit more house-leaning than I like, and they use automatic shufflers throughout on the ships I've been on. That said, it was still a great time.
The drink card is a bit difficult to earn on shorter cruises. If it's a 3 day cruise and you don't earn a drink card until night 2, there's not much opportunity to use it.