I took Tai Chi Chuan for a semester in college. It's a very calming exercise, like moving meditation really. More so than yoga (for me anyway.)
It makes you more balanced mentally and promotes physical balance too... but I wouldn't call it a big calorie-burner
If you want to feel "zen" and de-stress, then go for it, but don't expect it to crank up your metabolism much. At least, the version I took was not high intensity. Perhaps there are other styles that are?