Thought I would update this as nearly 3 years on my Sigma 120-300 2.8 has now gone elsewhere (to MPB Photographic, where else!), it's been replaced by this...
After the initial buzz of f2.8 at 300mm I quickly realised the 120-300 wasn't quite quick enough in the AF department for my needs, I fairly quickly regretted letting go of my Nikon 80-200 AF-S (which is essentially like gold dust these days), particularly when the Sigma started playing up on a regular basis. To Sigma's credit they replaced everything but the glass (so AF motor, lens mount etc) at an extremely reasonable cost, but by that point I had lost interest even though it was was now working perfectly.
Over the past few weeks I had a good look through my last 5 years worth of pics, I preferred pretty much everything from my '80-200 era' (my pics have got very safe in the past few years) so got in contact with MPB and worked out a pretty reasonable trade.
The 70-200 will get a proper workout next weekend, but I'm already amazed at how much smaller it seems than my old 80-200 (and how much lighter it is compared to the Sigma!), and an hour messing around in the garden with the cat confirms the AF definitely has that subtle edge that will make all the difference at the circuit.