I already have a 70-200f4L IS and a Siggy 120-400 OS, so I doubt the new 70-300L will find its way into my bag anytime soon. If I were starting over, the 70-300L would make a dandy lens for the outdoor photog. Now, if the new lens were a constant f4, it would be on preorder right now... 
My son has a 70-300IS. I've always thought that if Canon would fix the rotating front lens (with ring USM), get the IQ at 300mm sharp and boost the build quality a bit they would have a great lens. It looks like they did all that and more (weathersealing, 4 stop IS, and white paint
) with the 70-300L IS.