I went to pilot training twice. The first time, I had little or no prior training, and the second time I had a decent amount of time built up.
The first time, I attended Army Initial Entry Rotary Wing flight school. I had all of 2.7 hours of fixed wing time under my belt, which didn't really help a whole lot. But one thing did help me alot. I grew up around aircraft, and just knowing what a localizer was and being familiar with the terminology made it less confusing to learn. Stick-and-rudder wise, I started at zero. We had one girl in our class who was a CFI in helos, and she didn't have too many problems. Learning the "hands-on" part was the hardest aspect for me, especially during instrument training. I can honestly say I worked my butt off in Army pilot training.
The second time around, I went through Air Force SUPT, and I had about 400 hours total when I showed up at Laughlin AFB. About 50 of that was C172 time that I got through IFT. The other 350 was logged in UH-1s and TH-67 helicopters. The academics were very easy for me, because it was all the familiar stuff. The stick-and-rudder part was pretty easy too. There were things I had to learn to do that I had never done before, mostly in the arena of acrobatics, but after a few tries, that was pretty easy.
Perhaps the thing that prior flight experienced helped me the most was in the instrument training. Aside from a few Air Force rules that differed with the FAA methods (the AF changed to the FAA method right at the end of pilot training with a re-written instrument manual), the flying was relatively easy. Granted, you still need to pay attention and apply yourself and fly that airplane well. But compared to learning instruments without prior instrument time, it was very easy.
The hardest thing I had to deal with was flying the Air Force way. Each branch has it's own regulations on top of the FAA regs. They also have their own "techniques" (sometimes called "tech-cedures" because even though they are techniques, people get upset when you don't do it that way). Incorporating those regs and techniques was the thing I worked at most in pilot training...and losing all those old Army habits.
All in all, I can say that having prior instrument experience was perhaps the #1 greatest thing that helped me. That, and I believe that civilian and Army pilots get better instrument training. In the Army, we routinely did partial-panel, flying with a mag compass only, and we did emergency landings in the soup as well. In the Air Force, I never saw partial-panel, mag compass navigation, or any of that other stuff that I hated so much in the Instrument phases of the past. That said, I think it made me a better IMC pilot and helped me master a scan, which in turn helped me in the Air Force program.
Bottom line: you're prior training will help you SOME in the VFR training. But I have to say....flying around in a pattern with 11 other aircraft at 200 knots can only be experienced in Air Force UPT. You're greatest asset, with your prior experience, will be your ability to talk on the radio, have some situational awareness that the other students won't have, and you'll have an IMC scan so that you can happily fly on instruments all day while your classmates are still trying to figure out how to maintain altitude in level flight.
That being said, don't assume that you will just walk in there and be #1. You still have to work for it. Fly lazy, and the IPs will know it, and you're flying will suffer. Honestly, MOST people with prior experience do fairly well in UPT. SOME do not, and I honestly believe it's because they just didn't apply themselves. They felt like because they had all this extra time, they could just breeze through. If you do get a slot, still work hard. It will pay off in the end with a great assignment.