I have been shooting with my Canon 70-200 f4 for a while now. I really love it and it's damn sharp! But I have been wanting to try out a Sigma or Tamron 70-200 f2.8. so now i have this in my bag.. a Sigma 70-200 f2.8 HSM Apo Ex.
I was out trying it a bit now in fairly bad light.
Here's a picture (ISO-1000 f2.8) when the camera was working the servo.

I look forward for trying it out more and see which one is the best student choice :)

Have a nice day and shoot RAW!