I've got about 750 miles on my ShockStop stem and I like it a lot. I'm not looking for a "boing-boing" sensation. I would be extremely disappointed if it did that. I just want the stem to take the jolt out of hard edged rock/gravel/chipseal. It does that extremely well. Does it move? I don't know and I don't care. What I do know is that it takes the jolt and vibration away. The elastomer wedges are designed to absorb those bumps, not to bounce me back. To my mind, that's perfect!
If you're looking for more bouncy movement, I might recommend one of the systems designed to do that -- Future Shock or Isospeed or one of those old fashioned forks with the exposed spring.