I received a group of these bumpers about a year ago and find that they are like hard plastic, knobs are too short and they are so slick they are the only thing my dog has trouble holding onto when running.
I've been happy with both EZ-Rotational and Neuman & Bennetts, but the only Lucky Dog I've had essentially melted down in the bed of my truck. Maybe too much UV, maybe bad chemical reaction with the other bumpers or something else they share the truck bed with, but it's the only one of the lot thats surface has gone gummy.
The first bumpers I bought were cabelas. They are 3 years old and have cracked on the seams. They are hard as brickbats too. Lucky dogs have held up, but seem to get dirtier than others. They are also really hard in the winter. I won't buy anymore white luck dogs. I might buy black or orange though. Gundog supply has a good price on bumpers and ships with a good throw rope.