I do yoga, and recommend it on many scores. Not only does it help with your physical flexibilty, it also has a tremendous effect on calm and focus, dispersing stress and promoting physical and mental discipline. One of the basic tenants of yoga is a focus on breathing, and it promotes a steady, controlled, rythmic breathing as the basis of good yoga practice. This has obvious flow on benefits for diving, IMHO.
I can't really gauge the impact its had on diving. I noticed a marked improvement in air consumption when I began it, but as I started doing yoga at the same time as I embarked on a heavy cardio fitness programme and took up running as well, how much is due to the yoga and how much to the cardio work is difficult to determine. A well balanced fitness programme combines elements of cardio, strength and flexibility training.