I think you probably know my experience with the Sigma 300mm EX HSM f/2.8, but I'll say it again.
I bought mine from LCE Southampton during an event that they hold every year in the Spring at a local hotel. I was in the dilema stage of Sigma f2.8 or Canon f4.
Both Canon and Sigma were attending the event, but only the Sigma was available to see and try. In the end I took the plunge and opted for the Sigma. It looked great and I loved the feel.
My first outing was to a rallying event and then on to airliners. The optics were excellent. The results were superb, exceptionally clear and crisp so on this basis I couldn't fault it. Now for the cons.
The focussing was very poor, slow with a tendancy to hunt which was really annoying and a very common occurance.
Mine then started to fall apart. The mount close to the drop in filter become lose and them elements inside the lens started to drop down on to the optics. LCE assured me with was unusual and gave me a new one. Once again great results, but I'd lost confidence. I took it back to LCE, compared it to the Canon f4 L and decided to switch to Canon.
As a summary, I would say the Sigma had edge on the optics as comparing my results the Sigma's look slightly sharper. There's also the benefits of the 2.8 which you could use at Ice Hockey too. Having said this the autofocus was a constant pain, and the build on mine made me lose confidence in the lens. I'm happy with the Canon but not estatic, the autofocus is superior, it's much lighter and more portable, but the results are not quite as good in my eye.
My version of the Sigma has now been replaced by a newer version which Sigma advise is better. I may take a look, but definitely would check out the AF
before doing anything.