They are very good for grip strength. However, I'd recommend you have a somewhat strong grip first, because some of them are very hard to close.
And if you get the beginner gripper chances are you'll be too strong for it in no time.
I mean some of the strongest people in the world have competitions just to see who can close the #4 once.
So I'd probably get the Sport or the Trainer model.
If you want to strengthen your grip through other methods, I suggest plate pinches, wrist rollers (which can also help strengthening your shoulders, Farmers Walks or simply lifting heavy weight without straps or static holds.
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