Owned two different ones. I had the Nu Ball, which I rarely used and ended up giving it away so someone here. I worked okay, but then I got my second one which was more of a pro shop model. I got a good deal on it after Nationals from Frankie May. They didn't want to ship everything back, so a lot of equipment was being sold at good prices.
Even with the "pro" model, I realized that I rarely used it. Ended up selling it for a decent profit though.
So in short, I honestly don't recommend buying one.