Update October 11, 2014: I broke down and bought a pair of Yoga Jellies and used them for the first time last night. They work, but considering how graceless I am at yoga the moving of these appliances during a session is a bit annoying. Still, my knees seem to like them.
Yoga is such a simple practice that you really only need a mat. Other tools you use like blankets, straps, and blocks you can improvise if you don't want to spend the coin. Still, the sporting goods industry is always looking for things that you "must have."
Ironic considering the yoga lifestyle is all about living modestly whether it's getting by with less possessions, eating wisely, etc. On the other hand, if you suffer from Arthritis, Carpal Tunnel Syndrome or if you are heavy and have bony knees (guilty on the last two counts) this product might be worth the investment.