The 40mm was Maitani's special baby. He designed it just because he wanted one. As I recall he said it was one of the three lenses he took with him when travelling. That would be the best endorsement any piece of gear could get.
Mine is a lovely little lens, and it is hard to separate the optical performance from the esthetics. The balance and handling of the camera fluent. It is as sharp as any of the well acknowledged Zuiko's. But truth be told, I find the FOV to be a bit disconcerting. 40mm is the vicinity that Olympus put on their rangefinders and it doesn't seem much different from 35mm. But for me 35mm just seems to be the "right" FOV. That's just a preference and many folks would point out that the 40 is much closer to the standard focal length for a 35mm camera.
I have a 100f2.8 that has never been used. One reason is because that is an awkward f.l. for me. The other is the 90f2. That is a much more versatile lens, and is among the most hightly regarded pieces of glass ever made. Their price reflects their worth, but both the 90 and the 100 seem to be going for about the same amount on the auction site.