I love this thing because you can do push-ups on it (either side up) to add a unique challenge to boring pushups and strengthen your stabilizer muscles. You can put the flat side down and do crunches on it. You can put the round side down and stand on one leg to really strengthen your ankle supporting muscles, and many, many more options. I love it.
Rated #1 in Strength Training
This is great for conditioning, especially for the core. It can be used with the ball side up, and can then be flipped over so the flat side is up. Because of this, there are countless different exercises possible.