I have and use one of these with my players. I use it with shots where topspin is not the goal. Find it really good for using with players that struggle with overheads as they have to be more precise to even make contact with this. I also don't worry too much about the direction of their shot just the relative clean ness of contact. I find it gets them hyper focused on the ball and when changing to the racquet they smash better than normal. The same could be said for first serve practice. Always important to remember with these types of things that they as training tools to complement training techniques you already use( they are not a cure all)
Head PT57A S.E 3 Ti Radical paint job.
Experimenting with strings @ 48lbs