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Just wondering, it steel shafts, how big of a difference is there in regular flex and stiff flex?

Well there's potentially a pretty big difference.

The flex affects your ball flight trajectory, accuracy, distance and the feel.

Too stiff:
-low trajectory
-loss of distance
-fade/slice
-non-solid feeling

Too flexible:
Opposite of above.

I reckon there's more difference in steel than there is in graphite. It feels that way to me anyway, although effectively there should be the same amount of difference.

Id say there is actually more in a graphite shaft. My reason being is they tend to kick more and playing say a R flex when you should use S flex you are goin to have a heck of a time to get it right. Where as I can get away with any steel shaft from S down to A possibly L never tried.