But Neuro, I'm in the market for a 400 and this seems like a great price on what everyone says is a great lens. I agree, the 100-400 is more compact to carry & more versatile, being a zoom. I also understand you have a 600 f/4 at your disposal. But for a guy like me who can only occasionally rent a 600, already has a 70-300L, and would like something in the 400 range, this seems like a good buy. Finally, I was recently speaking with Greg Downing from naturscapes.net at the NANPA Summit where he gave two talks on BIF shooting. He carries the 600 f/4 and the 400 f/5.6, insisting in his talks that primes are the only way to go for BIF because they focus so much faster than zooms.
Can you add your thoughts, please? I'm a newbie to BIF shooting, but having a whole lot of fun getting into it.
The 400/5.6 is an excellent lens, no question. For BIF, you'll be shooting at shutter speeds high enough that IS isn't needed.
For me, it's a question of compromise, exactly as you say - flexibility, IS, and convenience. With a 70-300L, a 400mm prime makes sense - assuming you'll have time to switch lenses. Not sure, but I expect Greg Downing may have a body for each of those primes.
The 100-400 does fine for focusing on BIF, IMO. Not a great shot, but as an example here's one I took yesterday. The camera was hanging from my BR strap when I saw this red tail take off for a short flight from one tree to another, so this is a snap-shot - grab camera, raise to eye, focus and shoot. It grabbed onto the bird desipte the trees in the background.