I have had the 120-300 f2.8 out of a few outings now, so can report on it a bit more thoroughly. The lens itself is a non-DG sigma 120-300 f2.8 EX HSM. It is a heavy thing, but balances well with the 1DsII body, and is comfortable to hand-hold. Zoom is internal, so the lens handling is unaffected. My copy has a fairly stiff zoom ring, but I would imagine that will loosen a bit with use. As it is, zooming whilst holding the camera to my eye is possible, but a bit awkward.
It seems to focus on static objects quickly and accurately - no sign of back-focus that has been mentioned in some reviews, and the AF speed isnt bad. Using servo AF, I was getting about 60% of flight shots of fulmars in reasonable focus (good enough for web and small print viewing), and probably 10% sharp enough to view as a large print (A4 upwards). I would expect to get a bit higher sharp rate with my 500 f4.5, but I'm more practised with that lens, and the birds are further off, which makes their relative speed a bit lower as they fly across your view. So with a bit of practice, I hope I can get a higher sharp rate with this lens too.
Static shots on the tripod were fine - using a heavy duty ball head the camera was stable despite the weight. Long lens technique was needed though to make the most of the lens's sharpness. As you would expect, narrow apetures were needed to get any significant DOF, but you could get some great compression effects.
The downsides are that, due to its weight, once I've finished practising with it, I will only take it with me if I'm definately planning to use it or am not walking more than a mile or so with the kit, much as I do with the 500 f4.5, so it wont be the most used of my lenses. (And also, I cant claim that I 'need to cover that zoom range' any more to justify a lens purchase.
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Here are links to a couple of images taken with the lens recently.
http://www.worldphotographyforum.com...hp?photo=37498
http://www.worldphotographyforum.com...44&ppuser=1512