These were bought for my OH who is a former international swimmer. For him, the idea was to build maximum muscle during a swimming workout, but they are also useful for improving technique. The Speedo brand costs a little more than others, but I see it as an investment in quality. The elastic feels very strong and gives a tight hold (adjustable, of course). I tried them, too, and the effect is a bit like wearing flippers on your feet, i.e. if you do it correctly, you catch and hold more water, which makes your stroke more powerful and, consequently, more work for your muscles. I can see how they would help with technique because if your hand enters the water incorrectly, it's possible for it to slide out of line, so these paddles definitely make you concentrate on your stroke.
One aspect that is not well understood is that the paddle size has nothing to do with your hand size. They all fit all kinds of hands. The paddle size relates to the intensity of the workout; large will give you the highest impact muscle workout. A serious (and effective!) tool for swimmers at all levels, but to be used with caution at first. I certainly had a fairly stiff back and shoulders after using them for just a few laps. Also, if you are swimming in a public pool, be aware of proximity to others; remember, your hands now have a hard edge!