Warning, I am a raving fan of the Canon 70-200mm f/4L IS lens and have been using it since it was introduced. IMAGE LINK: http://rpcrowe.smugmug.com/Pets/Holly-Doodle/i-fzCffTf/A
I use the 70-200mm f/4L IS lens in conjunction with my 17-55mm f/2.8 IS (on a pair of 1.6x cameras) for all sorts of photography, general, travel, and some limited sports and wildlife. I use it hand-held very frequently in all light levels and will often carry the two cameras and two lenses for 8 to 10 hours at a stretch...
Since I carry this lens on a second camera everywhere I go, weight is quite important. The f/4L IS lens weighs 1.67 pounds (760 grams). The Sigma 70-200mm f/2.8 EX DG APO OS HSM weighs 3.15 pounds (1430 grams). Other f/2.8 lenses in this focal range are also heavy. The Tamron SP 70-200mm f/2.8 Di VC USD f/2.8 VC weighs 3.24 pounds (1470 grams). The Tamron 70-200mm f/2.8 Di LD (IF) Macro AF weighs 2.53 pounds (1150 grams).
Any of the above lenses are considerably heavier than my 70-200mm f/4L IS lens.
My 70-200mm f/4L IS lens can easily produce selective focus and the bokeh is just beautiful.
It is my favorite portrait lens...IMAGE LINK: http://rpcrowe.smugmug.com …ional/n-LPGK4/i-3dkpMt6/A
And my favorite dog photography lens...IMAGE LINK: http://rpcrowe.smugmug.com …UE-CALIFORNIA/i-Bg2xLJ6/A
It is also a great people lens...IMAGE LINK: http://rpcrowe.smugmug.com/Other/Istanbul/i-NvgkVpH/A
And, believe it or not, it is one of my favorite landscape lenses...IMAGE LINK: http://rpcrowe.smugmug.com …ERT-SOUTHWEST/i-PDKNtwk/A
I would guess that I shoot about 40% of my general, travel and portrait images with the 70-200mm f/4L IS lens and it is one of my favorite all-around lenses...
If you absolutely need f/2.8 (I don't) and if you don't mind carrying twice the weight (I do); then the Sigma might be the lens for you. However, I replaced the 70-200mm f/4L non-IS lens with the IS model as soon as the IS model was introduced. I have never been the slightest bit sorry that I chose that lens...