On my 5D Mark III, it's like a "baby - L" lens; sharp, great color & contrast from f2.8 - f8.
I test shot it against my previously owned 50 (1.8, 1.4, 1.2,) canon 35 f2 IS, and sigma 35 1.4.
The 40mm holds up really well against all those lenses. But it was much better on FF than on my 60D.
I was about to buy the 24-70 L II but decided to hold off for now, especially since I'd shoot primarily between 30-50mm; the pancake does that with low distortion! If I need a more durable versatile lens, I can still myself eventually getting the 24-70. In the meantime, I will use the money saved on a 2nd body, like a 6D or even a fuji x100s possibly.
I agree with neuroanatomist - when I borrow my friend's 70-200 L II, having the 40mm is such a convenience that gives me a great range while still maintaining mobility. Definitely get it!
Great pic Dylan! I actually used the 40mm on my 5d Mark III for a weekend at Disneyland and the beach, and it never bogged me down!