I agree with all the points stated above as I also have the IS version. When compared to my friend's non IS, my images are slightly sharper..but both lens are good lenses, and IS come in handy when you need it for shooting stills in low light situations. With the 550D the crop factor will make it a bit of a longer lens (about 112mm to 320mm full frame equiv), and it can also make a good 'macro' lens when zoomed in and at the closest focus distance.