Wes would likely have it as complete immunity.
As the GM whom probably makes the most common use of it, I treat it as a very good resistance that usually gets overwhelmed over time/prolongued assault. That way, I get to narrate exposure to psionics and present it as a danger that the players can react to, much like a protective barrier would on a power armor or a ship. If the psionic is powerful, he or she may eventually get through.
This only applies to psionics whom include for target the machine bearing the PSC, or its occupant. However, the PSC isn't as effective against indirect means of psionic attack like using telekinesis to lift metal girders up to then throw them at a power armor; but the PSC would hold up for awhile against - say - telekinetic attempts to forcibly dismantle a power armor.