The flex of your ski is best determined by your weight, how aggressive you ski and what conditions you ski. If you are more aggressive, or heavier you should be on a stiffer ski because you are putting more pressure on your skis. If your ski is too soft for you, you will notice a lack of responsiveness.
Very Soft skis are best for true beginners or children. They are very forgiving, and easy to learn on.
Soft skis work best for beginners and mellow intermediates that want a ski that is very easy to control at slow to medium speeds, that will require less energy or technique to make the ski react when and how it should.
Medium flex skis work best for skiers that will be skiing at a variety of speeds, and offer some stability at speed, and will be easy to control at slower speeds.
Stiff skis are most responsive at higher speeds, which need speed and technique to react when and how they should.
Very Stiff skis are for hard chargers or the heaviest or most aggressive skiers.