If you've got the ~200$ to spare, I'd go for it (In fact, I did).
I've been using both the 50 1.8 and the 40 on my 6d and I've been using the 40 more and more, though I haven't done any direct comparisons.
I bought the 40 a couple weeks ago because I wanted to take photos of my new baby, which means taking lots of indoor shots of the friends and family crowding around him. I find the extra 10mm makes enough of a difference in this regard, without causing too much (if any) distortion of features.
Furthermore, I do find the construction to be better to the point of making it more enjoyable to use (focusing, more specifically). Which is why I haven't been changing lenses back to the 50mm when I'm done with indoor photo taking.
Also, and I'm channeling my father here, 40mm is a great focal range for taking "people-in-front-of-landscapes". In the minimal amount of use in this regard I've had at this focal length over the past few weeks (walks in my neighbourhood), I tend to agree.
How are you guys liking this lens compared to the 50mm 1.8 canon? I'd like to get the pancake but if it isn't as sharp or produce comparable images, I feel like it isn't worth it but that is me.I like the pancake better (build, bokeh, AF), but 50/1.8'II seems to be a little bit sharper at f/2.8 when comparing at 100%.