medium vs stiff shaft drivers ... curious of how each affects ball flight ?

From the research I've done, seems the stiffer shafts are designed for players with faster club head speeds.    But beyond that, I'd like to know what difference the stiff vs regular driver shafts have on ball flight (example, will a stiff shaft make an unherent  slice better or worse) or have some other affect of ball flight ? I'm not a super fast ball striker, but I don't like whippy or overly bendy shafts ... thx

A stiff shaft is generally designed for players with swing speeds between 90-105mph. If you club head speed is lower than 90mph, a stiff shaft will likely cause shots that are too low with not enough spin resulting in loss of distance. It will also make the club feel more harsh and "boardy" and can also cause shots to fade or slice. A regular flex shaft will be much better suited for a player with this swing speed and will provide optimum launch conditions resulting in greater disance, and the shaft will behave properly through impact squaring the clubface and making shots straighter and more consistant (many people beleive that a stiffer shaft than recommended will result in greater accuracy but this is often not the case).

thanks man - appreciate the explanation

Just to add, briefly, it is not the overall stiffness of the shaft that primarily affects ball flight, it is the shaft's bend profile.

Here's a good run down on how shaft fitting works: