My take on shafts.

Shafts are made to flex within certain parameters. Shafts that are flexed over or under these tolerences will not take full advantage of the shaft. If a person with a slow swing speed, chooses a stiff shaft, they wont be using enough of the shafts "sling shot" and thus not maximizing their distance. If a person uses a shaft that is to flexible, he will bend the club to far, and loose accuracy. (Think of it like swinging your club way past parallel in your backswing). Finding a shaft that best fits your swing speed will maximize your distance, while still giving you complete control over the face of the club; trajectory, and spin.

The higher the swing speed, the stiffer the flex.

Also, shafts loose their resilience over time. My driver (400cc Hippo TI) shaft is S flex. Since the club is so old, its probablly more arround A (senior/ammetur flex). I demoed a R7 with S and XS shafts, and they felt nothing like my driver, "whippy wise".

Make sure you demo a good quality club too. It's best to demo clubs from a retailer when they visit your golf course or driving range. Using the demo clubs that these places "buy" to demo isnt the best choice sometimes. When a retailer of a club I want to use announces a visit, I make sure to keep a note of it.

Demoing a club that comes direct from a Ping or whatever trailer, makes all the difference in the world. You never end up demoing clubs with mat burn marks, or sky ball marks.