This thing ROCKS! I keep it in the car and play around with it every time I drive. It's great for those days when you just need to feel the burn, or for a nice warm-up on your way up the canyon. My only wish is that BackCountry would carry the other strength levels. This is the easiest and it would be nice to be able to push myself a little harder.
Bottom Line: figure-four out of the cave without worrying about crushing your skull.