Looka,
I have the same lens. It comes in a decent duffle bag that can be carried around, but definitely not good enough to be checked in. It does have a casing type of a coverage, so its not too bad.
AF speed is pretty decent and both D70 and D200 did not hunt on this lens in a non-contrasty scene. I had better luck with D200 in tracking birds and one can easily zoom in and out which comes with technique.
Now IQ wise, this is a sharp lens even when wide open, plus it has its own weird quirks too, but overall a superb lens. My only issue was it has squeaky autofocus and its currently in repair w/sigma. It is a superb lens overall, but I am sure the prime lens can be better. As far as the price is concerned, I thought the sigma is around 5.1k in b&h while the 500mm is 7.1k. The versatility of being able to zoom from 300-800mm itself is pretty amazing.
The bigma is heavy - about 15lbs and even though I use a 3030/3021pro combo, I wish I had a better mounting plate. I also carry a nikon 8-24x25mm binocs for quick composition before un/mountaining the lens to get an idea of what to expect.
check my site
http://emaging.smugmug.com under birds section to see some of the shots that I have take, plus I have tons more in my drive.
good luck and hope Sue gets to respond

Havent seen her in a while here.
Hi, I am thinking to shoot birds and wild life soon. My current
tele is Nikon AFS 300mm f/4 good for cage shooting I guess. I was
looking into this Lens Sigma 300-800 which you are using. I will be
happy if you can enlighten me how you pack the Lens, bag carrying
it and how is the AF speed? My local saleman recommand me the Nikon
AFS 500/4 which is about the same price, what do you think of it?
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