My DH does it fairly regularly, and I used to do it more often before I started running. It's very good for people who might have joint issues. My opinion is that you get out what you put into it. There are people in the class who just kind of float around, and then there are people who really try to perform at a higher cardio level. We have one instructor who makes everyone work extremely hard (she loves intervals!), and we're really huffing and puffing. Don't think that because your heart rate doesn't go up as much (it has to do with the water pressure), that you're not getting a good workout IF you're really working at it. Make all your movements big and try to go as fast as you can.
I, too, like the feeling of being more graceful in the water--I feel strong, long, and lean.