Haven't tried one but have thought about getting one. They were £25 on Amazon a couple weeks ago. Seems like a lot but if it works then it's worth it.
Yes got one and think it's a good bit of kit. Well made and useful to keep connected through the swing. I did try and make a Kaymer style tennis ball one (dog ball on a rope/lanyard) but his forearms are way bigger than mine and a tennis ball too small!
I take it to most range sessions and hit a few with it to feel what I need to do and then the rest of the session without it.