...Lens, that is. The original Pentax SMC 40mm f2.8 lens is a classic lens with a well-deserved reputation. It's angle of view is not quite as wide as a 35mm lens, but wider than a 50mm normal lens. It's very compact - about 3/4" long in front of the body, making it a very compact package when paired with a camera like the Pentax ME. I have tried other "pancake" lenses, such as the Minolta 45mm 2.0 MD and the Konica 40mm 1.8.
I've not tried the Nikon 45mm 2.8 GN nor the 45mm P lens -- the original is a rarity, and the newer AIS version is out of reach. Both lenses command a premium price these days. I liked the Konica 40mm 1.8 -- it's the fastest of the pancake lenses, but I didn't really care for the Konica Autoreflex TC camera, as I am not a fan of shutter priority. I sold off all my Minolta gear a while ago, but I am sure I have negatives and slides shot with the 45mm Minolta lens, but none are scanned to show here.
The Pentax 40mm lens is highly desirable, as it's very compact, and the DSLR folks have been driving the prices up. I was able to find a nice example online at KEH for a bit over $100. I really like it on my Pentax ME -- it makes for a lightweight and very compact unit to carry around, and is perfect for street shooting. I've used it in a variety of situations, and mostly with ISO 100 film. A film like Tri-X or Ilford HP-5+ should be perfect for all-around street shooting with B&W film.