I've got a couple now and wonder how I managed without them. They're spring-loaded so they adjust to fit most size pipes. They keep the wind from blowing ash everywhere and reduce the chances of getting burn-out. Plus, they're handy for covering a loaded bowl so you don't get ash or tobacco in your pockets. As an added bonus, they're helpful at keeping snow and rain off a lit bowl.
The only thing to watch is that they get VERY hot when you're smoking, so take care not to brand yourself with one.