Simply put....much better and consistent ball striking
I went through the iron fitting process with Jason Franklin in Santa Monica. He was quite easily able to identify issues I had with my old set of clubs. I'd been hitting an older set and the shafts weren't nearly what I needed given the torque in my swing. We ran through a bunch of different shaft and head combinations and settled on callaway apex irons with project x 6.5 shafts. They're awesome.
My first practice session with the new clubs actually seemed to invite me to swing harder with no fear of the shaft falling out of alignment on my downswing. The flight path is a deep and penetrating ball flight and I couldn't be happier. 8 irons are flying 180 yards into the wind. I had complained that my long irons didn't seem to go much further with my old set. I'd max out around 210 and couldn't get deeper with my 3 iron to fly further than my 5. Jason helped me decrease my spinrate and the new 4 iron was short hopping the 225 sign at my practice range (into the wind). I'm stoked.
If you're serious about the game of golf and want to see the potential you have unlocked go see Jason and let him pick a set for you. He's good. You'll enjoy the product and it'll pretty easy to understand why fitted clubs are such an advantage.