I used to use very stiff Prostaffs which were great for my serve and volley game, but when I started using more flexible frames, I found much greater control over my groundstrokes. Now I'm devoted to racquets with a flex rating of around 63 or lower, but I'm also hooked on heft. It gets tough for me to play well with a frame that's lighter than around 12.5 oz., especially up at net where I really need some stability.
Seems to me that more traditional strokes can benefit from racquets with some flex while players with more modern strokes and the angular hits they make on the ball can often benefit from frames with more stiffness. Flex and static weight are basically the two "deal breakers" for me; if one of them isn't right, I can't really do much with the racquet.