No real experience with the 80-400 vr except to look for a used one on
e-bay now and then.  They can go in the 800 USD range as most pros have
gone on to the newer 200-400 AF-S and are flogging the old VR off.
General opinion is very good glass, very lightweight, well built with
the non-S AF being very, very slow, but a great lens for the money!

It is not an AF-S lens, so it's not blazingly fast like the 70-200mm f/2.8G
ED-IF AF-S VR Zoom-Nikkor.  I use a TC-20EII with this lens with quite good
results. The TC does make the AF on a D70 and even a D200 noticeably more
sluggish though. VR does help a bit if you push the limit of the reciprocal
in focal length for shutter speed. Otherwise you don't really want to use
VR when your shutter speed is fast enough. I believe the 80-400 was the
first gen VR, the 70-200 second and now the 200-400 f/4 is VRII. But this
later lens is in a whole other ballpark, size, weight and $$ wise.  Since
you have the Sigma, what is the real or major reason you are considering
this lens?

FWIW, I've found these reviews to be pretty good:
http://www.naturfotograf.com/lens_surv.html
I agree with his reviews of the 70-200 f/2.8 VR and the 200-400 f/4 VR at
least. :-)
--
Ed Ruf (Usenet2@EdwardG.Ruf.com)
http://edwardgruf.com/Digital_Photography/General/index.html

I'm not a professional photographer, nor do I play one on TV.  Far from
it, I am a rather hamhanded amateur whose enthusiasm probably gets more
in the way than it helps make up for a lack of real knowledge of
photography.   That said, I was sucked in by this lens, and it stays on
my Nikon D70S more than any other lens in my bag ... except when I
leave the bag home and just take the camera and the 18-200 VR lens
instead.

I do a lot of motorcycle and car racing photography just for my own
pleasure.  The 80-400 VR stays on 400mm almost all the time, since as a
spectator I can't get to a lot of the relly good photo opportunity
spots on and around the racetracks.

Anyway, here are some of my results with the 80-400 VR; almost ALL of
these photos were taken with it, but some (a few) weren't...

http://tinyurl.com/qj7zn

http://tinyurl.com/zkocf

http://tinyurl.com/qv2oa

Hope these might help you make up your mind.  I love the lens; just
wish I were a better shooter!

--
Tim

I have had this lens for about two years, using it for everything from
landscapes to aerial photography. I have even done some close-ups with
it. It is not especially noisy, but it does not have Silent Wave,
either, which means that focus is pretty slow. It seems to focus more
quickly on the D200 than on the D70. Manual focus is a poor option
because the focus ring has enormous travel.

It is reasonably sharp, but it is no 80-200 f/2.8 VR, either, but then
that is probably the sharpest zoom in Nikon's lineup. It lacks a
certain crispness, but it is capable of producing very nice results
with careful handling.

The lens is very heavy. The tripod mount is awkward. Nikon warns that
you can pinch your hand when zooming it, but you would have to be a
real clutz to do that. This lens is unsuitable for sports or any other
sort of fast action. It is just too slow and too heavy. Not the lens I
would choose to grab a running cheetah. On the other hand, it is okay
for slower creatures such as orcas, sea lions, and bathing eagles. It
is extremely good with flowers and it does not have an unpleasant
bokeh.

You can shoot handheld, even at 400mm, down to about 1/16 second if
your hands are steady. At that speed I ruin about one out of three
shots, but I am a bit shakey. You will want to turn off the VR when it
is on a tripod, but it is okay on a monopod. On a tripod it just
chatters a lot and drains your battery.

It is a sturdy beast, capable of surviving drops onto rocks and
concrete. I have used mine in a typhoon with no noticeable damage.
Having said that, I suppose that no one would be interested in buying
mine used, eh? :-)

I'm not happy with mine.  I am an amateur bird photographer and this is the
only lens I use for my hobby.  I post my images at alt.binaries.birds if you
want to see what I'm always complaining about.  I've taken over 7,000 images
with this lens on my D70's and the "sharpness" has much to be desired.  I
believe this lens should perform way better for being $1,500. I was fiddle
fartin around with a D100 with a similar zoom range Sigma and, even though
it was heavy, the auto focus was perfect and the images were sharp. I think
that lens was about $500 to $600 dollars cheaper also.  The  80-400 vibrates
badly and makes a disturbing noise while it is auto-focusing.  When Mrs.
Purse-strings allows, I will probably get a 300mm f/2.8G ED-IF AF-S VR and
hopefully a 1.4 teleconverter(if it works with this lens) to replace this
80-400.  On the bright side, many people will find the "sharpness" adequate
and maybe even good.  It is relatively inexpensive and its zoom range covers
a lot of ground.  But.......I would never recommend this lens to anyone
serious into demanding photography.

Ilaab