Daddario Varigrip

Out of other similar products I've tried, the Varigrip does a slightly better job. It's made entirely of plastic, but doesn't feel too flimsy in your hand. The rubber padding for each finger as well as the palm holds nicely and won't slip too easily.

On the downside, it is a little noisy. The springs and plastic -combo cause a minor screeching sound when pressing the grips down. There really isn't terribly much room for adjusting the tension, either. And finally, turning it around to supposedly harden your fingertips as advertised is a joke. There's just some little nudges on the plastic frame you're supposed to press your fingers on, and that supposedly simulates guitar strings - utterly pointless.

I don't regret the purchase, I use it as a stress toy at work - but no one really needs one of these to develope finger strength.

There's really not much to say about this product. The good people at Planet Waves have put out a device some say is designed to substitute some hours of practice for minutes of pshysical stretching.
I don't see this as a substitute, but, it is quite good for those minutes you spend on the underground, the bus or waiting for a friend. You normally lose that time and, of course, you cannot take out your instrument and start practising in the middle of the street or on the train. So, that's when this toy comes in-hand. You can reach for it inside your pocket and start stretching. This won't be as useful as an hour practice but it will work out the muscles in your forearm and your fingers and enhance their endurance when you play for real.
There are also different levels of hardness in the response of the device to the pressure your fingers make on the grip.