RPM is a telfon/delrin type plastic. It's a "slick" self lubricating type plastic that is rigid enough to do the job, but typically flexes a lot more than the parts they replace. Which usually leads to a lot less breakage of arms/towers, fewer bent hinge pins, fewer broken axle carriers... things like that. The arms give upon impact.
The only downside is that the arms and towers give quite a bit. Where most stock parts would bust and your day is over, you will find on rare occasion that the RPM parts flex so far that you get bent shock shafts or bent axles.
But, considering the abuse they absorb, a rare bent shaft or axle is much more acceptable to a basher like me than busted parts.