While researching the D100 and 10D, the 80/100-400 range was the lens I knew I'd buy first.
I ended up buying the 10D and the 100-400L, but I can say some thing from what my research. The 80-400 is a quite slow focusing. It's intentional in its design. Not only is it not AF-S, it one of the slowest AF lenses Nikon makes. I've also heard mixed things about the quality control on the lens (same applies to the Canon.) Check out these reviews:
http://www.kenrockwell.com/nikon/80400vr.htm
This review doesn't think the AF is that slow:
http://www.naturfotograf.com/VR80_400_review.html
Most of the people I talked to (and what I read on dpreview.com) considered it quite slow.
You can also look here:
http://www.outbackphoto.com/reviews/equipment/lenses/mylenses.html
and:
http://www.outbackphoto.com/reviews/equipment/lenses/ronreznick.html
(at the bottom.)
The VR is the first generation VR, and isn’t as good as the 2 newer VR lenses. It works well, I’m told. But not on a tripod and under some situations not as well.
I own the 10D and the 100-400L F4.5-F5.6 IS USM. This is a wonderful lens. It's the lens I use 95% of the time. I find it to be very crisp and very fast & quiet when focusing. No CA and other distortions/defects (that my untrained eye can find.) Due to quality control issues, I wouldn’t mail order this lens. Some of them are duds and some are amazing. I would buy it from a place I knew I could return it… maybe many times.
IS is amazing. I can take shots at 400mm 1/150 (even 1/125) and still get the shot. Sure, the f5.6 sucks and I have to shoot at 800ISO often to even get that shutter speed, but neat image removes the noise and the shots still look good. I wish it was faster... but then it would cost even more and weigh a lot more too
Other than weight and cost, I only have one complaint. And I think it’s more the camera than the lens. It really bugs me how every now and then it just suddenly huts all the way out to infinity (or min focus, I don't know) and then seeks to the right focus. Some times, that precious second or two costs you the shot.
If you want examples of this lens, check out my gallery (both pictures use the lens) or these links. The first three pictures were shot at 400mm (not include 1.6x crop) and 1/300 of a second or less. The next 3 were shot a less mm, but still low shutter speed. I was about 8 feet away from the Heron. The last was 400mm, but I don’t remember the shutter.
http://www.marx7.org/~dsmith/eric/woodpecker1.jpg
http://www.marx7.org/~dsmith/eric/heron_house1.jpg
http://www.marx7.org/~dsmith/eric/heron_house2.jpg
http://www.marx7.org/~dsmith/eric/heron_at_grate1.jpg
http://www.marx7.org/~dsmith/eric/heron_at_grate2.jpg
http://www.marx7.org/~dsmith/eric/heron_at_grate3.jpg
http://www.marx7.org/~dsmith/eric/blandings1.jpg
That last is an endangered species, the Blandings Turtle. Protect by the UN and by state law. It’s a once-in-a-lifetime find, I feel very lucky.
If you have any other questions, feel free to ask here or by PM. I don't always check this forum but I'll try to watch out.
Eric