A lot of talk goes on about the out of focus rendering on a lens. This is a shot I took recently with my new 85mm f/1.8 lens wide open. The OOF highlights are perfectly round - something you don't see in many lenses... I often wonder why not, since the blades themselves aren't even in the picture.
Then, there is the new Panasonic GF1. While I admit I love the out of focus rendering, it doesn't come close to circular blur wide open except in the centre of the frame (taken with the 20mm pancake set to f/1.7):