STM is "the thing that isn't USM", right? Like on the c and other super-cheap lenses?http://photo.net/equipment/canon/lens-motors
Ring Ultrasonic Motor = Fast, silent but quite 'fidgety' focusing with full-time manual focus.
Micro Ultrasonic Motor = Slow, loud focusing, no manual focus in autofocus mode.
STM = Not either of the above.
"So that the STM can also be quiet full-time manual focus, plus ultra-compact, it is a technology looking forward to future developments."
So probably faster focusing that micro USM, and slower and smoother focusing that ring USM.
For me this is a great 'going to the pub' lens. Even the little nifty fifty makes a 5D look too much like a professional camera and starts people getting antsy when you try to take pictures. A slightly wider and really slimline lens is awesome.