I purchased a TUSK last summer, because it was only a couple dollars (and I think I needed that extra $$ to get free shipping). It's metal, looks better than stock, but now has me sketched out after reading this thread. Looks like I might be ordering one of these poly savers myself. Good info guys!
Delrin is pretty hard, better than aluminum or the steel stocker, but the polyurethane ones absorb the impact, they are harder than rubber pretty much like a skateboard wheel. Plus they are made by pouring the material into a mold shaped by the case, so they fit perfectly into the case, so when there is an impact the shock is evenly distributed, hopefully saving your case. Same reason you want your brain bucket to fit snug.