Stiff shaft v regular. My opinion
With regard to Drivers I have tried both the Nike VR Pro STR8 fitted with a Voodoo shaft in both a stiff and regular in both 11.5 degree driver 9.5 degree heads over an extended period. The regular gives more carry and allows a controlled relaxed swing whereas to get the best out of the stiff shaft you have to swing with confidence and commit to the shot to get the best out of the shaft.
Trajectory wise I have found that I get a slightly higher flight with the regular shaft as it's easier to launch the ball in the air with less effort. With the stiff shaft I get more feedback, control and rarely slice or hook the ball. Distance between the two shafts is marginal.
In a recent trial between shafts in one of the golf monthlies they selected three players and gave them a selection of driver shafts to try. My conclusion after reading the stats where at odds with that of the magazine as the majority of the shafts tried fit most of the players with marginal differences in spin, carry and roll. There were one or two shafts that obviously didn't suit the players, (this would be obvious on trying a driver), however in the main most of the shafts tested were comparable and for the every day golfer wouldn't have made a huge difference to his day to day game.