Can't speak for all commissioning routes, or for all the services, but I know that civilian flight experience had exactly zero effect on getting picked up for pilot out of NROTC.....it wasn't even something they asked about. I think that might well be different on the USAF side, and maybe some other programs. I was a commercial/IR ticket holder at the time. As for actual usefulness in Navy flight training......it helped me get through primary in the T-34C with good enough grades to get jets. Once I started intermediate/advanced jet training, it had basically no value, the flying was so different and you were flying so terribly much faster than in a Cessna/Piper, and doing quasi make believe "tactical" things too such as formation flying, dropping little blue bombs, and in a very loose definition, dogfighting/ACM/BFM. Long story short, I knew people who had backgrounds that ran from no previous flying experience, to Alaskan bush flying, to regional airline pilot. When we got through the first few weeks of T-45 flights, we were all on the same level, doing something completely new. At that point, success or failure was largely based on your ability to learn quickly and improve within the time allocated. If you didn't, you got booted. I knew 1000+ hr civilian former regional pilots who got booted, just like I knew folks with zero prior time who rocked the program.