I fly gliders and powered and would def say that gliding helped me when I went over to the dark side. However previous posters have a very valid point in that military flying isn't just about pole skills; it's about being able to operate the machine as a weapon.
I would say that initially someone with a couple of hundred hours in a glider and with XC experience would fare better than someone who had only been pax on an airliner before; but once the basic pole skills had been learned and you went off to FJ that the difference would probably have been eliminated.
The other thing that helped me when I went powered was my years as an avionics guy in the mob. I knew what all nav and radio nav kit did and how it worked before I sat in the cockpit.
Just had a look at my logbook and I went solo powered on my fifth flight and finished the whole shooting match well under the min time as I also did with my night and IMC rating. That's not due to any super piloting abilities on my part, more due I'm convinced to my past experience as a glider jock and instrument guy. (And also the excellent instructors I had.)