I'm on my second set of YakGrips because I love them! After 3 full summers of paddling a couple times per week the rubber wore out and the grips started slipping on my first pair.
They slip onto the paddle shaft after you wet it (hard to get all the way on the right, wider shaft end) and stay in place until the rubber wears out. I saw a review on Amazon that said they slip; when new, mine barely move at all on the shaft.
I find these help with paddling comfort, which reduces fatigue. If you have large hands they are especially helpful. My fingers no longer wrap way around the paddle shaft, making finger release on the forward stroke easier. I tried paddling gloves for a few excursions, but prefer YG's.
I highly recommend this for kayaking.