I purchased this lens about 8 months ago as my first “L” glass – since then I have added the 24-70 f2.8 Canon (without IS) to complete the range 24-200 mm at f2.8. I really can’t say enough good things about this lens, though in retrospect I might have bought the 24-70 first as it is a bit more versatile. The build quality is terrific and the IS is VERY helpful especially at the longer focal lengths. The f2.8 also allows you to get a teleconverter to double it to 400 mm (though I don’t have that as of yet). Though I’m sure the f4.0 is a great lens, the extra stops on this lens allows you to take existing light pictures in amazingly dim conditions. One of my primary uses for this lens will be school programs as it will allow pictures without flash in many of those conditions (especially after a body upgrade to push the ISO a bit – but that’s another story).