You wouldn't think this little bit of foam between your thumb and a soft rubber grip would make much of a difference but you would be mistaken... They do wear out, but, I probably get over a years worth of riding abuse to these before I feel the need to replace them. I would recommend applying "weatherstrip adhesive" to glue them in place as I'm not sure the adhesive on them will hold up over the long term.
I put grip donutz on every bike I ride. They are small and simple but make a huge difference when riding by eliminating blisters on the inside edges of your hands. An added bonus is that these last fairly long even when left on while washing the bike. I highly recommend these to everyone who rides.
I was always a bit of a skeptic when it came to grip donuts. I'd never had much of an issue with blisters on the thumb knuckle area so I didn't really see the point. After switching to some gloves with very minimal protection/padding in that area, I found myself getting blisters. I decided to give these a shot and sure enough, they do exactly what they're supposed to. They've worked out nicely and made longer rides much more comfortable.