I'm firmly in the GPS camp, properly used they must lead to more time being able to look out.
Did Rod ever publish the article mentioned on PCAS etc?
I've just got a Zaon MRX to experiment with but yet to decide how useful it is in practice. Even though it doesn't show non-transponding traffic I'm hoping it will be at the least a reminder that there are other aircraft around in the vicinity.
Look out is all very well but there are many blind spots and always the geometric fact that the plane on a collision course is the one least likely to attract your attention.