Summary: I must have gotten a bad lens based on these reviews but then again maybe they don't have to look at thousands of images at 100% in a photo editor. This is the worst lens I've ever owned. It is soft, soft, soft. Even my cheap 50mm 1.8 is significantly sharper. One reviewer said it's sharp on all four corners; not mine. It is embarrassingly fuzzy on the corners at any aperture and the edges as well as the middle too and forget about zone focusing.
Summary: Canon generally makes excellent lenses, but my copy of this lens is awful! Even at f/8 with a fast shutter speed and low ISO, images are blurry when blown up to 100%. It also suffers from severe CA, particularly in bright settings. Yes, this lens is inexpensive, but I can't recommend it at any price. You would do much better to save your money and buy a better lens. Canon should pull the 28mm f/2.8 off the market.