For bang for buck.. cant go past Sigma EX.
i have the 70-200mm ex and had a 24-70mm ex.
got rid of 24-70mm due to the 82mm filter size. but in performance you can't fault it and changed to 28-70mm ex older version with 77mm.
I have a big investment in 77mm filters
You will be pleased.
to me...
F2.8 is important for the dim light shoot. not for the extra f stop but for brighter image in view finder.
In most situations I can use flash.
Detractors say its softer wide open compared to nikon f2.8 lenses but i shoot manual
mode with F4 and shutter speed for handheld depending on the focal lenght, negating this softness. The nikon speedlight just work wonders.
Ever try to AF in dimlight with f4.5 or f5.6 lens you will know what i mean.
I am not sure if this the best way. but it works for me.