Bruce,
To me I couldn't tell a difference either. I used to use RPS strings exclusively (because that's what all of my guitars were shipped with) until I bought plain slinkys once and figured what the hey... I don't break strings and I couldn't tell any difference sound or playability-wise. I just use plain old hybrid slinky 9's now.
Oh yeah, I probably wouldn't have bought plain slinkys except that not everyone local carries RPS strings.
fred