In the best of situations, this product does what's advertised and helps you build core strength and develop the correct timing for hip rotation. It can be a great ab workout. However, I believe their are some cautions.
The Amazon video shows the old blue version which, at least for mine, closes with velcro. This was a goofy system and it seems that the new yellow version has a better closure system. (I'm assuming here as I don't have that one.) The problem with the velco version is it's difficult to size the belt since you have to arrange everything so there is the right amount of overlap between the male and female velcro parts. Too much overlap and it doesn't align properly and too little and there is not enough grip. A buckle type arrangement is a better solution if it does indeed hold.
Hold is the key word as, due to the size of the fins, they take quite a bit of force as you rotate. If the belt isn't tight enough it won't provide the proper resistance. One reviewer had problems and I wonder if swimmers who lack a little of nature's padding around the hip area (lucky people) may have trouble getting it snug enough.
I can imagine that swimmers who are not at least intermediate level may not have enough speed and power to generate a rotation due to the dramatic increase in resistance. This will slow you down somewhat so if your stroke can't abide any reduction in speed your legs may start to sink. However, adding fins should solve this problem and I'd recommend coupling them with this device. Just use the fins to lengthen your line and kick gently so you don't let the legs take too much load off your core.
Also, the belt (the blue style) takes a while to dry and usually has to sit in the empty bathtub overnight.
Still, if you can get it adjusted properly, I think this is one of the best training devices I've used. When you take it off you can really feel your hips rotate. If you rotate too late, you'll feel your hands getting trapped under your hips and make the correction to insure you've turned to the side during the end of your pull (the push and transition to recovery.)