What do you folks thing about the Powerball gyro exerciser ?
http://www.dynaflexpro.com/
http://www.powerballs.com/
I've seen folks around geekhack claim it helps with RSI, I have one and it's fun but can't say it really helped.
I'll quote Zappa again ... "And it'll cure your asthma too!" ...
Seriously though it seems like it's at least not bad for you like those finger weights, but some of the claims they make on their site are pretty ridiculous. It might help some people by building up auxiliary muscles, but don't forget that RSI is all about certain muscle groups (and tendons, joints, etc) making the same repetitive motions over and over again. It's not often related to weakness in muscles or groups, it's a form of abuse. That said there's not really exercises or exercise gear that are going to stop that from happening as far as I'm concerned. What will help is it you take regular breaks from the repetitive motions that are causing your hands stress (and possibly stretch or use one of these balls or whatever curing that break to break up the repetitive part of those stresses).
The only real solutions for RSI are ergonomics and trying to lessen the amount of the stress involved by reducing the stresses involved -- i.e. lighter keyswitches, better ergonomics on the board like keywells, better wrist support, etc. Also relieving real stress (not muscular stress) can make a big difference as well.
My condition, which is arthritis, is a bit different. I do have to do regular exercise with my hands and fingers to keep them limber, typing alone isn't enough. The exercise I've been prescribed to do doesn't involve extra resistance on my hands/fingers though .. it's more about range of motion and the like. Lots of stretching too.
As Billbin says, you can get the same beneficial effects by moving your hand around. On an RSI related approach I very much believe that is true.