I'm a big fan of thick-handled exercise implements, such as fat barbells, fat dumbbells, thick handles for pullups, and so on. They force you to grip tighter in order to hold on to the implement, which both increases your hand and forearm strength but also causes you to squeeze a lot of other muscles tighter, too. So it's no wonder that a friend got me a pair of Fat Gripz for my birthday.
Cost: $39 a pair, plus S&H.
What are they? Fat Gripz are pliable plastic 2.25" / ~5.5 cm thick (closed) and 5" long / ~12.5 cm external grips you can fit over a standard or Olympic barbell, dumbbell, or other handle. Those barbells are 1" thick, so adding these makes the bar twice as thick. Additionally, although they "close" fairly snugly, they won't stay in place without a firm grip. So you must squeeze pretty tightly to keep them completely secure on the bar. That's a feature, not a bug - you're putting these on to make you grip tighter and work harder.
Why use these? To improve your grip, mostly. They also have the nice side effect of being a bit easier on your hands than a metal bar, so you're less likely to tear (or generate) a callous using them.
Any downsides? They fit a bit snugly to dumbbells, so you have anything slightly undersized (the neutral-grip handles on an Elite FTS rack, for example), they won't fit. Otherwise, though, they are an excellent substitute for a myriad of thick handled implements - you can get these instead.
Utility: 5 of 5. They're easy to put on, reasonably easy to take off, and work as advertised.
Quality: 5 of 5. These grips will stand up to a lot of wear-and-tear.
Overall: If you like thick-handled exercising, it's worth having a pair of these. They're very portable, too - you can chuck them into a gym bag and bring them around with you. Recommended.