I've has 1 of the non IS model, and 3 of the IS models. All of them were good, the non-IS was the sharpest. I ended up selling them because I had the 70-210mm F4L IS which was superior in image quality, and much lighter and easy to use. My old arms got pretty tired after carrying the F/2.8 around for a while. Now, I use primes when I need a wider aperture, F/2.8 is marginal in low light.


Is does a wonderful job of helping get quick shots off handheld. You must be careful and deliberate with non IS, and keep the shutter speeds up. When using a crop camera, I'd use 1/500 or faster.


This negates the usefullnes and expense of f/2.8 to some extent.


For about the same price, the f/4 L IS is a bargain.