They say ~ Treat your nails to the ultimate nail file - a crystal design from OPI. This is made from hardened 100% non-porous glass with an ultra fine, abrasive 240 grit surface etched into it and will help to prevent your nails from chipping and peeling. It can be cleaned with ease and comes in a handy carry tube for storage and protection.
I say ~ I've used cheap glass files before and they wear out, but this is different. This file is quite gritty and files your nails quickly, turning the excessive length in to dust.
I like how you can file all ways over your nails without damaging them. Unlike emery boards you can wash it and that restores it to the original grit. Only drawbacks for me are the price and the sharp end as being so clumsy I often have jabbed myself with it.