The "DO" label on this lens stands for "Diffractive Optics", and that's how it works according to Canon (diffraction). The fresnel likeness is how I prefer to describe it in laymans terms. Except this lens has little grooves on such a teensy level, they really do operate diffractively (is that even a word?).

My memory is improving a bit now, and the prime weakness I have with this lens is indeed when shooting towards bright light. Blue sky is fine, but towards the sun is difficult. The lens tends to put bloomy halos around bright spots, like the light leaking through the treed background in the sample crop above. Things get washed out easily when shooting towards very bright light.

Me, I use it to pick out architectural details mostly, but also for long distance shots -- being careful to shoot away from bright sky rather than towards it.

Cheers