Been pondering my long tele options over the last few months... A 100-400mm is looking very likely, but I'm left pondering, why would anyone but the 400mm f5.6 prime ?
Serious question, what does it offer other than being very slightly cheaper and lighter ?
Just another list of pro's for the 5.6/400 for ME:
* IQ (sharpness, contrast) because I want to use it with my 2x TC (mark 1)
* contralight flare rejection (assumed to be better due to the low number of lens elements)
* sturdy construction
* fast reliable AF
* built in lens hood
* I like the idea of a 24mm - 40/50mm - 100mm - 200mm - 400mm prime set since I startet photographing
The prime won against the 100-400 zoom because I really like shooting "perfect" primes. The 70-200 4.0 is an exception because it delivers excellent IQ (except direct contralight situations).
Sometimes I missed the flexibility of a 100-400 zoom because I like to walk with ONE lens / body.
Another point about sharpness/resolution: Under real world conditions the excellent IQ of the prime goes down to roughly 1 MPix when you do landscape photography - due to shimmering atmosphere.
If I were NOW in the situation to decide between the 5.6 400 and the 100-400 I would perhaps wait - if possible - for the mark ii version of the zoom. If it has similar IQ like the prime, a 4x IS system and sells for 2.5 kEuro it would be my choice.
Just my 2ct - Michael
PS: An example of 5.6/400 with 2x TC (mark i) from an HD video with 3x zoom (EOS 600D, 1 sensor pixel = 1 image pixel), effective focal lenght is roughly 4000 mm.