In reply to apd: Aha I have a powerball already, at least that saves a few bob!
It crunches and makes little popping noises from flexion, extension, abduction and adduction but not the other two.
I can do about three push ups before I can't get off the ground anymore! I tend not to do them that much because I'm crap at them. It doesn't seem to hurt my wrist at all. I've severely sprained my shoulder before and the pain isn't quite the same. It's more of a sharp ache (if that exists!). Oh actually that's why I stopped climbing one session because my wrist went when I was bouldering and I woke up the next day with a sprained shoulder which was a bit odd.
I started using it regularly for about three weeks (twice a week, so only six-ish times in total) and it started to niggle at my elbows so I stopped and started doing antagonistic exercises to stop them hurting. I can't do long on it anyway because I can only do ten-ish pull ups so each session was maybe ten-fifteen minutes long.