This buoy is alright except for one thing: it is slippery when wet which makes it hard to keep it between your legs during workout.  On the plus side, the shape of it is indeed very comfortable and natural feeling.  The fact that it is fillable for variable bouyancy seemed intriguing to me, but because it is so slippery, I found that filling it makes it only harder to hold on to.  Rather than concentrating on my pull, I was focused hard on squeezing my thighs together so the thing wouldn't slip out.  It was a nice idea, but somehow Speedo needs to figure out a way to make it a lot less slippery.

It is an awesome idea, but very uncomfortable when swimming.  The hard plastic hurts especially when completing flip turns. Maybe it could use some padding around the outside.