i have that setup, and it's not atrocious.. having a dedicated 100-400mm would obviously be way better, but i didn't have the money for both it and the 70-200mm f/2.8, and the latter's features were way more important to me, so i decided to make do with the 2x for the rare occasion that i need something longer than 200.


your colors and sharpness do suffer a fair amount, and you do lose a couple f/stops, but it's still better than any of the 70-300 lenses, and it's less gear to cart around.