Gamers? 5X faster loading times for infinity blade 3.
Sure. It's just that 99,9 % of iphone buyers play angry birds, clash of clans and farmville, and I'm not entirely sure how much better their experience will be with the new 64-bit processor. Especially when you factor in that Apple must provide a compatibility layer for 32-bit apps for years to come, so unless you got some "Apple special developer dude" bonus check, there isn't much incentive to migrate your app to 64-bit.
The sixty-fourth time's the charm: the benefits of Apple moving to a 64-bit architecture on iPhone was... *drumroll*
SLIM TO NONE!
Edit: I'll reiterate this as well: it was basically useless, but it was mandatory.