In reply to Boris\'s Johnson:
I've used the Gripsaver and Powerfingers. I prefer Powerfingers because you get 5 different resistances in one package, but one has broken already after a few months. I've had 2 Gripsavers and they've lasted substantially longer than that.
If I can work out where I got the Powerfingers, I'll send them an email, pretty sure I should be due a free replacement after such a short time. Doesn't mean I'll get one though.