Got the varigrip a few weeks ago, and after some days my fingers already felt stronger. It's also a nice warm up before playing an instrument (I play piano/violin), as the keys and strings feel lighter afterwards.
Overall the product is really good, but the only thing that sucks is that even though you can adjust the weight for each finger, it already starts to feel a bit light even on the heaviest setting. Then again, for the pinky it is quite heavy even when not maxed out. But still it would have been nice if there was a option to put it even heavier, especially for index and middle fingers.