Hello fellow Ausphotographers, just thought I'd help out where possible and do a brief review on these two lenses. I hope this helps with your decision.


Lens Sample Archives.
Canon 70-200 f2.8 IS

Sigma 70-200 EX HSM


Dimensions.
Data from DPreview.
Canon 70-200
51.9 oz(1470g)3.4 x 7.7"(86 x 197mm)77mm
Sigma 70-200
48.3 oz(1370g)3.4 x 7.3"(87 x 184mm)77mm

Sharpness Tests.

Canon 40d - ISO 100 - AV mode - Tripod - 10 Sec Delay - Live View Focus - Unedited RAW-JPG conversion.

At 2.8 the Canon clearly beats the Sigma in sharpness and contrast, however past f5.6 there is almost no difference. The Canon's colour reproduction is much better at all apertures, however some colour detail is lost at smaller apertures. Tests below.
70MM








200mm









Brokeh

Almost identical in every photo I took, Canon's contrast is still much better




Mount and Build Quality
The Canon's build quality and "presence" is far beyond that of the Sigma. It's mount is made of higher quality material and fits better, something important if keeping the rain out of the camera is needed.

Street Price
Sigma - $799US - $999 AU.
Canon - $1699US - $2599AU.

Verdict
With the price difference to large, amatures and semi-pros looking to shoot with a quality telephoto will find the Sigma more than enough. If your a good photographer, or just plain rich, the Canon provides some slight extra sharpness and IQ at a big price. For me, I use a tripod/monopod where I can, and have stable hands, so the IS feature will never come into the equation.

If your shooting soccer in a cave, and cannot bring a tripod because you and the other thirty photographers in the sardine can have no room, get the Canon!

The Sigma is half the price, but certainly 4/5ths the lens.




Disclaimer
Nikon photographers may get overly excited when you pop out the white lens. Use with caution.