If you play trombone in an orchestra, this is the best money you'll ever have spent.
Our section leader has been using one for as long as I can remember, and I finally decided to take the plunge and get one for myself. It's a godsend for those long periods of either counting rests or waiting for the conductor to finish working separately with the winds or percussion. Any time you're waiting - no more awkwardly holding your trombone, setting it in your lap, worrying about dinging it - just lock the slide and pop it on the stand!
I'm buying another one for my brother.