I bought this to have fun and work on my dolphin kick/butterfly stroke. This fin amplifies the power of your dolphin kick and in doing so helps train your body to do the dolphin more effectively sans monofin; I have definitely improved. I did find the material to be a bit stiff, but perhaps it will soften over time. You can also buy some sort of monofin aqua socks to minimize strain/friction on your feet, but I have yet to experience these special socks.

You can gain some impressive momentum with this item (my goal is to swim with dolphins), but given the amount of water you are moving with each kick, there is significantly more pressure on your legs and I have cramped given the additional strain - I imagine this will wain with  more training.

Overall, I am very pleased with this purchase - lotso fun.

This fin provides a good workout.  It doesn't make you fast straight out of the box, though, there is definitely a learning curve that comes with it.  The floatation of the mono fin is nice but not neccessary.  I would recommend this product to others not because it helps your dolphin kick but because it is neat to use.  Swim outlet carries the best Finis monofin Rapid if you are shopping around.

As a coach I use this fin in two ways. First as a training tool to build strength and endurance. Second to help teach older swimers kicking technique. The fin helps with the bending of the leg and whipping action in both a flutter and dolphin kick. With my water polo golies, we use it to help build leg strength.
The only problem is in putting on the fins, this is a bit awkward. Sitting on the dwck putting on the fins seems to be the fastest way. Except fot this I will be buying more of these fins for my teams.