The EF 70-200mm f/4L IS has stunning IQ and covers almost all conditions when paired with a camera with good high-ISO performance like the 6D or 5D MkIII. The reasons to get the 2.8 are if you're a wedding photog or a indoor sports photog or you're a bokeh whore, not that the f/4's bokeh is all that bad:
Merry Christmas From Animal Kingdom by
dcstep, on Flickr