CampX

I bought this lens, along with a 40D body and a bunch of other goodies this summer. After much searching, reading a ton of reviews, and agonizing over the choices in buying a long lens, I decided on the 400mm F5.6L. Definitely didn't go wrong.
I have a 70-200mm F2.8L IS which I had thought about just getting a 1.4X or 2X tele-converter for, but the reviews said that the focusing speed and IQ takes a big hit when you do so. The 400mm F5.6L focuses very fast, and I'd say my keeper rate, when I have done everything right, is probably 7 or 8 out of 10.
Being used to shooting with a F2.8 lens most of the time, one has to pay attention more with this lens being a F5.6. Most of the time, it's pretty sunny where I live, so ISO 200 or 400 works great, and I usually just leave it on F5.6, as the images at F5.6 are razor sharp anyways. I still have the odd 'ooopsy' moment when taking bird photos, when I have closed down the aperture to F11 and the bird is sitting still, then the bird decides it wants to fly away......blur. As for being hand-holdable, this lens is light (compared to my 70-200 F2.8L IS), doesn't fatigue the arm at all, and so far I have not used a tripod with it.
I find that this lens is on my camera 80 per cent of the time these days, and my poor old (actually like brand new!) 70-200 F2.8 L IS spends a lot of time at home on the shelf...makes me think I should maybe be selling it. All in all, I am loving this lens, great piece of gear for the low price of it.