It is a great lens, that said I sold mine and now use the 70-200 F2.8 with a 2x teleconverter, I sacrifice a bit of light, but not noticably so and in my view the 70-200 is a superior lens and I can achieve almost the same with a teleconverter on it, as I can with a proper 70-400 lens.
Ideally I would have both, but I would rather have another decent lens and a 2x teleconverter for the price of a 70-400. Just something to consider, but I am not taking anything away from the 70-400 which is great.