Please, when you post a review for a lens, state which camera bodies you used. I used a 5D with this lens.
I assumed that this was a good performing lens based upon the high review marks received here. After purchasing and testing 3 copies, and testing three other copies, I conclude that this lens cannot keep pace with full frame digitals. I was hoping to use this lens for available light indoor photography and DOF subject separation.
I am choosing the 70-200 f/4 IS instead. Photozone tests that model to be Canon's highest resolution zoom. Folks like the contrast, and it's supposed to have the latest & greatest stabilization.
The test was a controlled still-life setup, testing all three copies at the same time on a tripod-mounted camera. Tests were conducted wide-open because I want to be able to use the lens that way. Tests were done at the limits and middle of zoom range. There were inconsistencies between the copies, but all were soft. I later compared the best of the three to the so-so 100-400L lens with the longer lens wide open and the 70-200 f/2.8 stopped down to match. The result: the 70-200 & 100-400 produced virtually identical resolution (accounting for different magnification). Also compared was the most dependable copy of the three to a colleague's copy. His was substantially sharper at 70mm but softer at 200mm. Another test was between this 2.8 IS model and a store's f/4 non-IS model, and the f/4 model was substantially superior.
I would not recommend this lens until Canon updates it, and instead produce better images using the cheaper and smaller f/4 model.