I just picked up two used 40mm STM's for stereoscopic shooting. Yes, there are used ones out there already!
Mine will be on the market soon enough too. I fell head first for the novelty value when it was announced, used it all the time for a fortnight and haven't used it since. Right now it's little more than a functional lens-cap. But that's just me.
It's a really decent little lens for a great price. But at this focal length I'll keep reaching for the Sigma 50 f/1.4 or the 24-104 f/4is, and soon the 24-70 f/2.8II.