5k, the Flexi-Bar looks to be the same as the Bodyblade i use at the gym. It is great for working the rotator cuff and has helped me bring my shoulders back into balance and to prevent injury from my heavy lifting.
many tools have their place in the gym but some are realy bad for you or just don't do anything.
an item as light as the "shake weight" at 2.5 lb is going to be useless for working the targeted muscles except posibly for endurance in completing that movement
and the "scientific study" only compares it to a DB of the same weight and the homepage has no links to actual study results and conditions.