Mine love them. They chew them and when a little dust is produced they start to roll all over them. When any of the chins here get bored with them over time I sand the outside and produce a little dust and then they go to town on them again.
I'll have to try the sanding down, mine love them when they are fresh. Smokey has to wear an E-collar right now and cant dust, but he sure likes to rub his little face on his rock!