When I was a kid, we didn't have products like these that we could buy at the local sporting goods stores. We had to make them. My dad saw SF Giants Matt Williams warming up with something similar back in the early 90's and decided to make one, which he did with an old ping pong paddle and some shoe strings. It worked wonders with my fielding mechanics, which is why I was quick to purchase this product.
I started coaching a 9U baseball team about a month ago and there's a lot of kids who have never played baseball before, or never had coaches who worked with the kids. We're not those coaches, so we break down into groups, and one of those is for fielding practice. I purchased some sofhands trainers and some reaction balls and have used them (*separately for now) quite a bit so far. I have noticed some major improvements and the kids have fun.
I recommend watching the training videos on YouTube to get an idea on how they need to be used. Obviously you want to just roll the ball for a while until they get used to it, as well as make sure that they're fielding correctly, which is what this product helps with.
Overall, I highly recommend this product for anyone...ANYONE who plays baseball or softball. It will definitely improve your game. Also, try it out with the reaction ball once you get used to it.