[vachss]

I must have dumped more on Canon gear in the last few years than I did to buy my Z4, but do they send me a lousy discount code... NOOOOO. Of course, I'm not really looking for gear right now, but it's the principal of the thing....

Anyway, I actually like the 300/4IS quite a lot on the 5D2. It works very well with the 1.4x as well. I believe the 70-200II + 1.4x may produce results similar to the bare 300/4, but I'd be surprised if the 70-200 + 2x equals the 300 + 1.4. It's this latter combo of 300 + 1.4x that really needs comparison with the 100-400 since many folks are looking for the cheapest way to get a sharp 400mm.

My comparison:
1. The 100-400(@400mm) and 300/4 + 1.4x are about equally sharp - good but not perfect wide open at f/5.6, excellent by f/6.3-7.1 (assuming a good copy).
2. The 300 + 1.4 gives a true 420mm focal length while the 100-400 maxes out at a measured 380mm or so.
3. The 300 + 1.4 focuses significantly closer than the 100-400 and is very useful for quasi-macro - butterfly/dragonfly kind of shots.
4. All that said, the 100-400 is a zoom and if you are shooting a variety of different sized critters in quick succession (e.g. a zoo) a zoom is MUCH more convenient than a handful of primes.