I have seen quite a few forearm strengtheners on the market. Is any one better than the other in terms of results?
Gripmaster | Rock And Ice Forum
First, any type of grip trainer will be a very poor substitute to training on a climbing wall. I normally only recommend these gadgets for warming up or rehabilitating finger strains. However, they are certainly better than nothing if you can't get to a gym or use a hangboard. I recommend both the donuts and the Grip Masters for doing static contractions. There really isn't much difference between them. Don't pump them in and out too fast. Instead, squeeze them in for four or five seconds, release for two or three, then hold in again for four or five seconds and repeat until you get pumped. Train one arm at a time if you only have one device, although it is better to use two simultaneously. Use your fingertips and experiment with a half crimp and a full crimp. You can also try holding them above your head for a few contractions and then lowering your arm to shake out.
The gyrating balls are recommended for training the extensor muscles of the forearm. They can be a great way to avoid or rehabilitate elbow injuries, so thumbs up!
Get some Metolius rock rings. They are portable and better than grip-trainers as they enable you to do hangs and pull-ups.