I just got this thing yesterday and had a blast with it before practice.
I have a serious issues with kicking, i dont like to do it. but i could allready feel a pretty big difference after using it for such a short time.  I have a very strong fly pull but my kick has always lacked a whole lot. we did a set of 6 100 fly with fins and i was flying throught it, definetly got a lot more of my chest out of the water. after we did that set we did with out and I was cookin in that set, usually for me its tough to get through a set like that, but i could fell huge improvements in the overall quality of my stroke.

It does however fit pretty small, i wear size 13 shoes and it would be a strech to get a size 16 foot in there like it suggests it will fit. On the plus side a lot of the kids on my swim club with smaller feet can definetly use it.

I am very happy with the product and its a great tool for anyone trying to make improvements on dolphin kicking. I didnt notice that it was difficult to swim on the top of the water like the other review said

Excellent training tool for developing kicking technique as well as leg and core strength.  Forces body into proper alignment when training.  Fin size allows for good kick tempo and swimming speed.  Initially best used UNDERWATER due to surface resistance.  Surface training is difficult.  This fin is not practical as an "assist" for extended lap swimming as are other types of fins intended for use with flutter kick.  Excellent build quality although sizing felt less than true as due most fins.  Suggest purchase of at least one size larger than range that includes normal shoe size.  Use with fin socks to provide good fit and avoid chaffing.  Fiberglass blade is rounded and all edges are smoothed off.  However, does pose risk of injury to other swimmers if struck by blade.  Should not be used while sharing lane.