I was at a place that had them and used them, I was not fond of them for a couple reasons.
1) The strap you pull them with constantly gets in the way. One of the greatest skill the players will develop is how to avoid the strap (if not they will get tangled up).
2) The base (inner tube) is far too wide. The whole thing rests on an inner tube which is about 3 feet in diameter, and sticks out further than the actual shadowman. To make a tackle your player has to either step far to the side of the base or dive through the tackle, other wise his feet will hit the inner tube base.
Shadowman is a good idea, but these two flaws really get in the way and make it a poor tool. If you had the $$ (which is a LOT) the remote control devices used in the NFL would be far more useful.
Everything in life is either a tool to serve the Lord and do his work, or an idol to distract us from Him.
Our program has them - our HC spent in the neighborhood of $6k on them. I think we used them all of 4 times last year. I was not impressed by them. We teach hawk tackling. I think you could get the rolling tubes and have the same benefits at a fraction of the cost. My $.02.