Most players consider using longer shafts in their clubs at some time or another, believing that longer shafts equal more distance, although that isn't necessarily true. There are two ways to lengthen your shafts. You may add extenders to your existing shafts or you may replace your existing shafts with longer ones. The effects can be very different. Tom Wishon has written extensively about the effects of changing shafts in "The Golfsmith Practical Clubfitting Program" guide.
Shaft length is certainly a factor in how far you hit a given club. However, some clubs benefit more than others. Installing longer shafts in wedges and short irons will almost always result in more distance, as most players have little trouble swinging the shorter clubs. As you move toward the longer clubs in your bag, however, the increased length can actually result in less distance because lengthening your shafts alters other aspects of the club's performance.
As a general rule, lengthening any club's shaft will reduce your accuracy with that club. The longer the club, the more difficult it becomes to hit the ball solidly. This is especially true when the ball is on the ground rather than on a tee, since hitting the ball fat is more likely. Hitting the ball off-center with a longer shaft creates more unintended side spin.
Swing weight isn't an actual weight so much as a comparison of the difference in weight between the club head and the butt end of the shaft. It's expressed as a letter-number combination, such as C9 or D2. Lengthening the shaft increases the swing weight, which makes the club head feel heavier. It may even make the club too heavy for you, which will slow down your swing speed and actually reduce your distance.
It's a fairly simple matter to match the original shaft flex when installing a long replacement shaft. However, using extenders to lengthen a club will change the flex of the club, sometimes by an unpredictable amount. Not only does the shaft become more flexible than before, thus further affecting accuracy, but it lowers the kick (or bend) point of the shaft. And while the kick point change may only affect strong players with higher swing speeds, any player may feel that the club doesn't contact the ball as solidly as before.
Timing and Feel
In many ways, the biggest effect of lengthening a club shaft is how it affects your ability to swing the club. The combined changes in length and swing weight make the club feel different. In turn, these changes will affect your timing and even your mechanics, perhaps without you even realizing it. You may simply feel that you must move differently to get the club around. As a general rule, you should seek a professional's help when you consider lengthening your club shafts. It's the best way to ensure that you don't accidentally harm your game.