Is it Good Exercise?

Yes and no.
One of the responses I often get when people learn that I'm a ballroom dancer is "It's such good exercise!"
And of course it is. There are benefits to cardiovascular fitness (Viennese waltz, anyone?), balance, and flexibility.
But if you want to go beyond the basic "slow-slow-quick-quick" level, everything changes. You don't dance to stay fit. Instead you stay fit in order to dance.
My daily routine includes at least one long walk and a strength/flexibility/balance routine at home. Several times a week I do a gym workout.
I take a weekly mat class and have a private Pilates session once a week with a certified trainer who's also a professional dancer.
And I take a modern dance class that always begins with a fitness warmup.
It's true that dancing belongs to the realms of imagination and spirit. It is a doorway to another world.
But it also requires extraordinary physical ability. You're not going to soar and float unless you have upper-body strength, strong calves, and flexible quads and hamstrings.
Even something as apparently simple as a triple-swing basic step requires exceptionally strong abdominal muscles. If you've moved beyond the novice level of dancing, you need to learn how to use your stomach to move your leg. Not easy.
That sounds crazy, and it took me a long time to learn how to do it. (Bolero requires the same kind of connection between abs and legs.)
Cha cha and tango (and many movements in other dances) require you to be able to stop yourself with your stomach muscles.
And then there's your back and your obliques.
A good dancer is a physical package. Everything has to work together. If you're weak anywhere, it's going to show up on the dance floor.
All of this probably sounds like a lot of work. I'm not sure it is. "Fascinating" might be a better word for it.
Pilates is sometimes described as "intelligent exercise." That intelligence factor is what makes dance fitness so interesting. You're not just grunting. You're studying how your body works and celebrating tiny victories as your body organizes itself.
A friend of mine who majored in dance in college was required to take a kinesiology (anatomy of the muscles) course right off the bat. She was furious. The teacher, an MD, wasn't even a dancer.
Later on, though, she said it was her favorite course--she learned things she applied every day.
This "get fit first" principle doesn't just apply to dancing. When I lived in New York City, the YWCA used to advertise ski fitness courses. The idea was that you could prepare for skiing season by working the muscles you'd be using on the slopes. You'd ski better and avoid those first-day-on-skis aches and pains after a whole summer away from the sport.
Back to ballroom dancing. Certainly part of its fascination is that it's not any one thing. It's not just a fitness tool, or an art form, or an avenue to self-expression.
Fascination, fascinating. Dance.