Golfers who are serious about their equipment do tend to split into one of two camps with respect to this question. From my experience, those who feel the driver head is more important believe so because at one point in their golfing life, they have made a change to a different driver head and immediately noticed improvement. I’ve found the same thing goes for those golfers in the other camp – at one point in their playing life they have made a switch to a different shaft and noticed improvement.
For those who believe the driver head is more important, if you send me your favorite clubhead, I will install a shaft into the head, final assemble the club, send it back to you and I will bet the farm you can’t hit the club very well. Likewise for those golfers who believe the shaft is more important, send me your favorite shaft and I will put a driver head on that shaft, final assemble the club, send it back to you and I will bet the farm you can’t hit the club very well either.
The point is, both the clubhead and the shaft have to be well fit to the golfer for the entire club to perform the best for each golfer’s size, strength, athletic ability and swing characteristics. The most important specifications of the driver head for most golfers are the Loft, Face Angle and the head weight. If any of those three elements are ill-fit to the golfer, no matter how well the shaft, length, total weight, grip or any other factor of the driver are fit to the golfer, the club’s performance will be poor.
The shaft is a little different animal in that its full performance contribution to the club depends on the golfer’s point of wrist cock release and their downswing tempo. For golfers who have an early wrist-cock release and a more passive downswing tempo, the weight of the shaft is most definitely important, but the shaft flex becomes much less important for the full performance of the club than does the clubhead and its key fitting specifications. This is because the shaft flex requires a later wrist-cock release and a more aggressive downswing tempo before it really starts to show its influence on shot performance. As the golfer develops a later wrist-cock release AND a more aggressive acceleration of the club on the downswing, this is when the shaft starts to become more and more important to the total performance of the club.