The little brother - Canon 50mm f1.2L vs 40mm f2.8
The three Amigos: Pentax-M 40mm f2.8, Canon 40mm f2.8 and Contax 45mm f2.8
If you haven't read Roger Sicala's test on the Canon 40mm f2.8, please do read it. Roger is a great writer and his blog is always worth your time to read. I haven't used this lens much, but for those pictures I have taken, it's a great little lens and I like it a lot. If memory serves me correctly, the Canon 40mm f2.8 is sharper than both the Contax and Pentax pancakes, especially at the corners. It doesn't focus as fast as a USM lens, and is noisier too. I don't see how this lens, which was made for video using Stepper Motor (STM), the focusing noise will surely be recorded.
Should you buy one? Yes. Sometimes when it's on sale, it goes as low as $150, not much more than the Plastic Fantastic EF 50mm f1.8, but a much better build lens, and optically better. You won't be disappointed.
Bicycle - Canon 5D Mark II & 40mm f2.8 STM @ f2.8