I had tried the DuoBall friends and loved immediately thereof. So I've bought the DuoBall itself. I also use the already Blackroll and am very excited about this. The ball is perfect to place it at the spine, so the two muscle strands and thus to massage himself or to "bother" :) It is useful at the beginning to dose the pressure a bit, but it's faster than that you look at it, puts the whole weight. Beware of people who are very sensitive when it comes to pressing and massaging Musekeln! Whoever screams already in the normal massage, should not necessarily determine the ball once or start with something softer.
As I said, I find the Duoball great and versatile. It relaxes muscles and relieves tension and I feel like it ever so much better.