I love carrying the shorty 40 either in a walkabout kit as a pocket or back throw in with a longer lens (telephoto zoom or 135L). I recently traveled with the 24-105L, 135L, 1.4x and shorty forty. All of this went comfortably into a smallish sling bag and was a great combination. I did everything from landscapes to 3 portrait shoots on the trip. The shorty forty is a nice complement to the 135L for environmental portraits.
40mm is a nice "woods" length lens. Wide enough, and the sharpness is as good as anything I've got for things like that. It is also very sharp at minimum focus distance, which allows for some very crisp leaf or flower shots like this:Hanging Around
by Thousand Word Images by Dustin Abbott
, on Flickr
I do disagree with the reviewer as to this lens vs the 50mm f/1.8. The 40mm, while a slower aperture, is perfectly usable wide open, has much better color rendition, smoother transition to ooF, better build quality, and, of course, focuses much faster and quieter. IMO on a full frame body the 40mm is a preferable focal length (I prefer the 35mm range to the 50mm).