I switched to a bend shaft years ago and would never consider using a straight shaft paddle.  When you use a bent shaft and you are at the end of your stroke you are still pushing more less horizonal.  With a straight shaft at the end of the stroke you are lifting water with your paddle.  The difference is very noticeable when you paddle a tandem canoe solo.  In a tandem canoe when it is used solo with a straight shaft paddle the front of the canoe will lift with the end of the stroke.  The reason the front of the canoe lifts is because the back of the canoe is being forced down because the paddle is lifting water.    The lifting effect will be more noticeable the farther you sit back in the canoe because the front has less weight on it.  Then compare how much the front end lifts when using a bent shaft.  I think with practice a paddler can have an effective J stroke with either paddle.