I purchased the swimfin for my 5 year old grandson after hearing about it on holiday. As in the picture, it is fin shaped, comes in great colours and fits round the body with two adjustable straps.
Although he can't swim properly he is fairly confident in water, but with the swimfin he was off like a rocket and immediately swum a width, just like that. He is delighted with the idea of being mistaken for a shark and is looking forward to wearing it in the sea for maximum effect.
I'd say it does everything it says. Any criticism I have is about product information. I wasn't aware until after my purchase that it comes in different sizes/weights, and it was only by luck that we have the one we need. The backpack looked interesting, but it was a while before I worked out that it wasn't any kind of additional buoyancy aid, but just a neat way of carrying everything around. Information could be a lot clearer and more detailed, especially on the official website.
The sad thing is that he is not allowed to wear it at his school swimming lessons, not at swim club which cuts down his opportunity to use it quite a bit. I don't know whether this is because the teachers don't know what it does, or just a general ban on any kind of aid. Either way it's a shame as it does add fun and confidence to his swimming sessions.