I recently bought a 5D Mark II and it introduced me to the high ISO performance of full frame sensor that I had never seen in an APS-C body. I no longer shoot my f/2.8 L zooms wide open as much as I had done before. ISO 1600 is so clean and ISO 3200 seems better than ISO 800 on my 7D. I can't imagine you guys with newer bodies like the 5D III and 1D X. With the announcement of the 24-70 f/4L IS I am seriously considering selling all my f/2.8L zooms and switching to f/4 L IS zooms. 70-200 f/2.8L IS will make room for 70-200 f/4L IS, 24-70 f/2.8L will make room for 24-70 f/4L IS, I no longer use the 16-35 f/2.8L because 16 is just too wide. I'm giving up the f/2.8 zooms because I don't shoot at f/2.8 anymore and they are too heavy and I don't get any younger, I turn 28 in 2 months ! What do you guys think? Smart decision to switch to f/4?