Canon EF 40mm f/2.8 STM
The EF 40mm F2.8 STM offers excellent image quality in a tiny, well-made package. For APS-C users it provides a great carry-everywhere complement to a kit zoom, and for full frame shooters it's an excellent general-purpose 'normal' lens. The STM motor means that autofocus is almost completely silent, but it's not super-fast.
Canon SLR owners who want the image quality of a prime in a tiny, relatively inexpensive package.
Photographers shooting in really low light or aiming for maximum background blur. They'll do better with a faster lens like the EF 50mm F1.8 II.
Canon EF 40mm f/2.8 STM review
The Canon EF 40mm f2.8 STM is a very interesting addition to the Canon EF lens catalogue. The company's first EF pancake lens, it makes even the chunkier EOS DSLRs look and feel much more compact, and can turn smaller models into something you could more easily squeeze into a large pocket. After all, it's almost half the thickness of the already pretty compact EF 50mm f1.8 II. In this respect...