There is an almost organic feel when you're training with a kettlebell. The weight moves within your grip to realign itself with the forces that act upon it which means that the forces that then act upon your muscles are those of your bodyweight augmented by the physics of the kettlebell itself. The result is a more targeted, intensive workout that brings many of the body's supporting muscle groups into play for a transformational experience.
Extra Credit: 30 seconds rest between sets.