The 50 1.4 looked to be the way to go, but this at a cheaper price and similar build might be better for me. The tiny size means I'd be able to put a body with lens into my work bag or even the handle bar bag on my bicycle.
I have had the 40mm now for about 3 days, really fun little lens but at 2.8 the 50 f/1.4 beats it pretty solidly, 50 f/1.4 also has nicer contrast. That all said I think I will keep it around, it makes my 5DM3 nicely portable and the quality certainly isn't bad, just doesn't hold up that well against the 50 f/1.4 which isn't THAT much bigger but is almost 2x the price.
I have a few photos of it on my camera http://digital.photorecommendations.com/recs/2012/06/40mm-stm-quick-review/ and I hope to rent a T4i in the next week or so and test the full time AF with STM - for those folks who are buying the T4i I think it is a no-brainer.