I love this lens. It makes my 5D mark 2 into this light, take anywhere camera. The lens is extremely sharp, however the combination of my copy and my camera body required serious AF micro adjustment (-13). Before I did that AF was seriously out - the background would be in focus! My other lenses have not required major adjustments.
Color looks good.
While I find sometimes that the 40mm focal length is restrictive and makes me wish I had brought my other lenses, it's usually in circumstances that if I didn't have the 40mm, I would not have taken my camera bag due to the weight.