My impression from all the tests is that the f/4 IS is optically the second-best of the 70-200mm zooms (it was the best until the f/2.8 IS USM II showed up), and personally I would not "down-up-grade" to the 2.8 non-IS, gaining one stop of aperture and losing the IS and the superior quality. I also walk around outside a lot, so the lower weight of the f/4 is a good thing for me, too. If you feel that you don