actually, the 75-300 lenses from canon all have rather poor image quality, especially at the 300 end; AF is slow and contrast is poor; but it's still quite acceptable, only the 70-200 lenses typically give better optical quality--better sharpness, contrast--especially the f4L and f2.8L ones if you have the $$$ haha. or you can always try those from sigma, tamron, etc.
errrr i'm using a 28-200 sigma marco lens.. bought for $440. this lens is very gd to "me" and she produce nice photos in both landscaping, events and marco :lovegrin: love this lens. but in future will plan to get canon L lens. maybe this thread will not be helpful to u but juz share it with you.
found the lens both the III usm and non-usm to be plasticky and extend wayyyyy too much... for a 300mm it is light... found the optics to give less saturation than other lenses? anywae... if u getting it wld be for the IS
If you're not too particular about the optics the Sigma 70-300mm is a good working lens for BRIGHT SUNLIGHT only...the aperature range limits its use only for outdoor use. Ahh...then again, for the lenses you've mentioned their aperture leaves much to be desired in low-light.
For the macro function, I've haven't tried it out, but for macro only at 200-300mm range, you'll need to put quite a bit of distance between you and your subject (the lens sticks out like Pinocchio's nose). BTW, the magnification's only till 1:2 and minimum focusing distance is 0.95m.
With IS, U can compose with greater ease. The shakes are less visible while composing yuor image. Thus, less likely to go wrong at long focal lengths. For sports too... More easy to see the action thru the viewfinder. Just imagine the old type of video cams, those without the anti-shake system... u'll know wat I mean...
IS also gives u extra 2 more stops. can go as low as 1/60s at 300mm, handheld.