Some interesting comments so far. Some not so. One thing I know for certain from personal experience, frame flex is overplayed as a way to sell. Two guys in the local race circuit that were amazingly fast and won crits all the time, along with mountain bike races, were on Sling Shots. If any of you have ridden one of these creatures it is at first scary, but when your body and mind acclimate to it, all is well. These flexible fliers performed quite well under the legs of two seriously powerful riders and never seemed to hinder their performance.
Eventually they took their Sling Shots to national level races and found another level of talent out there, but they still faired well considering they had zero altitude living in their lives. Flat landers that rode frames designed to flex an inch or more laterally as well as vertically. It worked for them!