Sigma 120-300 f2.8 EX APO DG - Why buy one?

My 300mm f4 ED IF Nikon lens needed upgrading and whilst I would have preferred the Nikon 200-400 f4 VRII the price tag of £5000 wasn't realistic. So after much deliberation and looking at reviews the Sigma won the day. But how good is it?

First impressions

On receiving the lens I realised the photos don't give you any true impression of size: this lens is big. The front element is 105mm which, whilst on the subject, if you go for Sigma's recommended filter be prepared to add £100 plus to your lens cost!
Taking the lens out of it's box you get your second shock- it's heavy - 2.6Kg heavy and, until the new VR version is available you will probably need a monopod or a very sturdy tripod. Because I use a 2x converter on my 300mm, I purchased the Manfrotto 393 long lens bracket a while back and it certainly proved useful with the first trial of the Sigma.
The build quality is excellent. Everything feels tight but responsive with large rubber coated zoom and focus rings. On attaching the camera (Nikon D90) it feels heavy but manageable although I much prefer mounting to a tripod. Even the tripod mount feels solid (one of the complaints about the later 300mm f4 Nikon lens). The lens also comes with a very useful and sturdy soft case and soft lens cap cover.
Time to take the lens for a test drive and my choice of venue is Richmond Park and the Red Deer Rut. On arriving (6:15) things are still reasonably dark and very misty so a tough start for the lens. At 1600 ISO the lens performs well even down to 1/16th second and I know my Nikon 300mm just wouldn't have performed at all. Focusing is silent and quick and the zoom ring has a reassuring resistance to it. Once the light improves, colours are punchy, aberrations are kept well within acceptable limits and it's really sharp across all apertures.
Check out my Red Deer pictures. I have a busy couple of weeks ahead and hopefully will report back with more impressions later.