I personally prefer shafts that are somewhat flexible with just a bit of "springiness".  I think this makes it much easier on the muscles and joints. However, the current infatuation with carbon fiber  (primarily due to its lower weight) imposes a significant limit on this because of  the inherent rigidity of the material.   If you desire more flex in your shaft, seek out or order a paddle with an all fiberglass, s-glass, or wooden shaft.