I bought this lens on Ebay for $129 wanting a lighter, smaller alternative to my Tokina 28-70mm f/2.6-2.8 lens. I use the lens on my Canon 1D MKII camera which has a 1.3x crop factor, making the effective focal length of this lens 31mm to 110mm. Unlike the huge and heavy Tokina lens this lens is small and light and can stay on my Canon 1D MKII without the camera falling forward at rest. That alone was a pleasant surprise.
But even more pleasing are the pictures that this little gem produces. I've owned the Canon 17-40mm L lens as well as the 28mm 1.8, 50mm 1.4, and 85mm 1.8 Canon primes, so I do have some basis for comparison. All I can say is if you're thinking about this lens...buy it! Yes, the L series lenses have a better build quality and the primes are faster...but for general shooting this lens is a steal.
Forget all the technical mumbo jumbo. Buy this lens and see if you like it. I think you will.