If you're looking for grip-training tools, Ironmind has a line of hand grippers that set the standard. They are the Captains of Crush gripper series.
These grippers are famous for hand training. Unlike most non-name brand grippers you'll find in a supermarket, they take some strength to close. Originally they came in 5 strength levels, from Trainer (100 pounds of resistance) to the #4 (365 pounds of resistance!). More recently, they've added two beginner models and a number of intermediary ".5" versions to provide smoother progress from one gripper to another.
The manufacturing quality is high. They're uniformly excellent and durable. The all-metal handles are very resistant to wear. The springs will eventually lose strength with repeated use, but they're meant for low-rep strength work rather than banging out hundreds of reps in a row. If left in a damp environment, they can get a little rusty. But they're solidly constructed and you feel the weight of every penny you paid. $19.95 each may seem pricy, but it's a reasonable price for a high-quality item.
Using a gripper can be humbling. They really take a lot of strength and fair-sized hands. They'll be difficult for a small-handed person, easier if your hands are large. The handles are knurled, which provides for an excellent grip but which will tear the hell out of your hands. Gloves will only make the gripper more difficult to use, so it's chalking your hands or nothing. Nothing is not recommended.
I own and have closed the Trainer (100 pounds of resistance) and #1 Gripper (140 pounds of resistance); I've used a #2 Gripper (195 pounds of resistance) as well but have yet to close one.
These grippers really do set the standard. So much so, that if you close the #3 gripper or #4 gripper under verified conditions, they'll give you an official certification. The amount of strength required is high, so this is something for strong hands to aspire to. It's not a low-bar, feel-good certification. This sort of respect comes with all of the grippers. Tell someone who knows these grippers, "I can get 10 reps on my gripper!" and it'll mean nothing. Tell them "I can close a #1 Captains of Crush gripper" and they'll know you've got some good hand strength. As each number falls to your grip this will improve. They are a great training tools and a good measuring stick.
Quality: 5 out of 5. Rock-solid, and they'll only break if you deliberately abuse them, if then.
Utility: 5 out of 5. If you're training crushing grip or isometric grip like holding a bar during a deadlift, these are the tool to use.
Overall: If you're serious about crush grip training, get these grippers.