Just to let you know where I am coming from. I own the Canon 300mm F/2.8 II lens now. Before that I had the Canon 300mm F/2.8 IS.
The Sigma 300mm F/2.8 lens is not at it's sharpest at F/2.8. Post processing these raw files you can see the difference (it does not jump out at you) but it is there. It is usable, and is a good 2.8 shooter. But, stopped down to F/3.2 makes for an excellent photo.
All of these shots were taken in AF-C. I was checking the camera to see what I could get by with, and how accurate it was with moving subjects.
Here is a photo shot at F/2.8.
Pentax K-3, ISO 1250, F/2.8, 1/400 Hand held. The IS on the Pentax K-3 is very good by the way.
Click on pics for a larger view.
It is not quite as good as the Canon 300mm F/2.8 IS lens at F/2.8. But, my copy of the Canon lens was not at it's sharpest at F/2.8 either.
Here is a shot with the Sigma 300mm F/2.8 lens at F/3.2.
Pentax K-3, Sigma 300mm lens F/3.2, ISO 1250, 1/800
Here is a 100% crop with the 300mm lens
Pentax K-3, Sigma 300mm lens, F/5.6/ ISO 1250, 1/1000
Another shot at F/6.3.
Of course Pentax and Sony do not need OS. So, if a Canon or Nikon shooter I would personally pass on this lens. This lens starts to shine using it on Pentax and Sony mounts IMO.
Personally I do not like tele-extenders at all. So, how this lens performs with an extender is another matter. I refuse to use them. I will use them on the new Canon Tele-photo II lenses some. But, limit it to the 1.4 only, at this point.
So for what it is worth. Used with the Pentax K-3, the Sigma 300mm F/2.8 lens is a good alternative.
The AF was fast and fairly accurate. You are not going to set any speed records, but nothing to be ashamed of.
Anyone shooting the 300mm F/2.8 lenses knows they are the fastest focusing lenses in the world. They are blistering fast.
All the best, and good birding