I got rid of mine ages ago writing it off as a gimmick, though having had a number of wrist injuries over the years my physio told me to try a powerball out.
I’d be sceptical but she’s a hand & wrist specialist and understands to forces involved in climbing so I’m going to give it a go for strengthening!
I don’t have any way to get post (other than amazon lockers) so before I order one online I thought I’d check to see if anyone locally had one they want rid of?