As a middle-aged re-entrant runner, I used Galloway time-goal programs for a couple of marathons including my first successful Boston Qualifier. For the time goal races, I only walked through the water stops, that is, I didn't do anywhere near as much walking as he recommends during the race. I still use planned walking/hydration breaks in my long runs, though less frequently than Galloway calls for. During one training cycle using Galloway I ran a small marathon as a scheduled 26 mile long run in the build up, completing the race at a training pace and using the full up run/walk approach. The thing that most impressed me is how quickly I recovered from that effort.
I've since moved on to other training programs and have significantly dropped my race times in the last couple years, but still use some elements from the Galloway programs. As mentioned above, I still use the run/walk routine on most long runs. I use the high quantity mile repeat workout sometimes and I do one moderately long weekday run, which is something I started on those plans.
My evaluation of Galloway is that it's a sound way for a relative novice runner to build up mileage and complete a marathon. The run/walk approach certainly leads to faster recovery which is helpful to those who find themselves too beat up by a long run to effectively train on successive days. His claim that racing with a run/walk strategy makes sense for higher mileage competitive runners (whatever that means) is questionable at best.