I switched over from a 12.4mm 314/2, to the fat shaft about a year ago. I'm quite happy with the change, the theory in my mind was that I spin the cue ball too much and my ball pocketing wasn't quite where I wanted it to be so I hoped the fatter diameter would dial down the amount of spin I could put and help me find center ball a little better.
And that's exactly what happened when I made the switch. Today (a year later) I'm very happy with it and can't see myself going back to a smaller diameter predator shaft anytime soon.
The fat shaft does seem to "turn" the cue ball a little more when applying heavy spin, maybe that's the small amount of extra deflection that I saw on predator's website when comparing the two shafts. But I was able to get used to it fairly quickly.
Some one pocket banks that require tons of spin to miss the kiss become a little bit more difficult because I feel like I can't apply as much spin as I used to but it's a trade off for better ball pocketing and center ball control for me.
I use a 30 inch fat shaft with a kamui black hard tip.