Excerpt: This is a decent lens. It's tiny and it's very sharp. At f2.8 it's already sharp, and produces a nicer image than my Sigma 17-70 at any aperture. So much so I don't use my Sigma anymore. I use this more than any of my other lenses at the moment, I find it a very useful focal length, probably more so than 70mm for my style of shooting anyway.
Pros: Sharp, tiny, light, 3d quality
Cons: f/2.8 is simply not fast for a prime in this focal length, out classed by other primes
Excerpt: I bought my 40mm pancake lens with a (very) used MX body. I find it a good combination for street photography, as they make a lightweight combination and the 40mm size is just right for the sort of urban shots I like.
Excerpt: The SMC Pentax-M f/1:2.8 40mm is the original Pentax "pancake" lens. At the time, it was the only one produced (by Pentax) and was highly desirable for the M bodies. The combination made it the first "pocketable" SLR camera. (But, you needed a large pocket.
Pros: small, exceptional construction quality
Cons: difficult ot find focus ring when used to 'normal' sized lenses