I have done it but don't currently. I think it can be a very valuable supplement to your fitness program but, contrary to some overzealous proponents, it can't replace other forms of exercise for strength, cardiovascular or even flexibility training.
I find it very relaxing. While it's by no means taxing in any way, I think it helps clear the mind and help me focus on something. One of my favorite teachers had us do tai chi before taking tests