A few weeks back I got a foam roller from Amazon; it's been an interesting addition to my training. Mostly I use it to work on my IT band, calfs and hamstrings, though I suppose I'll be adding my quads sometime soon.
But it's painful, like a good massage is painful. And it's the first time I ever truly, actively felt a muscle knot go away. After I located it, I just rested my body weight on it and said I'd count to 70, bear the pain, and move on if nothing happened. Right around count 30, I was able to relax the muscles around the knot. Near 65, the muscle started spasming. At 80, the muscle knot was in its death throes. At 90, it had melted away, and felt... well... a little weird.
There have been times after using the roller that I've felt taller on my right side, if that makes sense. There have been times when I've been in enough pain that I've wondered if I'm damaging myself. But I've never felt anything like the muscle knot going away, before. I've given massages that have coaxed knots to give up the ghost. I've had massages done to me that have cured both a screwed up gait and what I thought was unrelated pain.
I can't say that I'll love my foam roller as much as my Garmin. But if I use it regularly and intelligently, it may be just as useful as, say, Gu. Which is pretty high up, just under proper shoes, good socks, and apparel. :)