What are they made of?  Is it really any different than a Promax copy?

The KAC grip is the real deal.
Promax grip is a copy.

KAC materials and design have been tested and proven in combat.
Never seen a Promax grip, have no knowledge of their materials, design or trials.

KAC will mount on any Mil-Spec rail system.  
No experience with Promax.  I have read that some copy grips do not fit Mil-Spec rails tightly.

It is POSSIBLE that either the KAC model or Promax model may fail at some point.
It is PROBABLE that the KAC model will not fail.

Not sure you could say that about a copy.
Might just depend how hard you're gonna run it.

I have a KAC and a ProMag copy.  The ProMag is a little longer, but both units are made completely of some sort of plastic.  Both units fit my Surefire KAC and MI #20 rail perfectly.

In my humble opinion, if enough force is applied to one's weapon to actually break or dislodge a verticle grip, I would rather have that happen then suffer possibly more damage to the weapon from the grip staying rigid against the force.

My point is, the higher quality (more expensive) KAC may actually be a liability.  I like the ProMag because it's arguably the "weak link" against extensive weapon damage.