Personally I have owned the 400/5.6 along with the following other solutions over the years
300/4 IS - awesome lens, great for frogs, dragonflies and small birds. Not so great with extenders.
70-200/4 IS + 1.4x - bare lens is very nice, mediocre with an extender
70-200/2.8 II + 2xIII - sharper than the 70-200/4, takes a 1.4x very well, but 2x loses a lot. AF is poor with extender.
100-400 - nice lens, but only slightly sharper than 70-200/2.8 + 2x III and AF is poor
Sigma 80-400 - extremely soft, not worth it
I absolutely love my 400/5.6 but personally I have noted that it's really a lens that a photographer uses - not a hobbyist. It has no IS and poor magnification, so in order to get the shots you really need to understand what it is capable of. Still, it is far sharper than any of the solutions above and has the best AF @ 400mm.
Although I love this lens, I am currently selling mine in order to fund a 200-400/1.4x. Ideally I would like to not sell it, but as nice as this is the 200-400 outdoes it.
Below are some photos taken with it.JSC_3936-Edit.jpg
, on FlickrJSC_4686-Edit.jpg
, on FlickrJSC_5053-Edit.jpg
, on Flickr