At this point in time, these are my findings regarding STM:
- slower focus than USM
- continous AF possibly in video, but not really smooth, compared to a little training and doing it manually
- lower noise, but still noise audible when using continuos AF, manual is still better for STM when noise is crucial
- good for ad hoc AF videos or travel videos, so at the current stage I do not think its of interest for pro video but rather for entry level (which does not mean, that we don't see STM capability on an entry FF in the future)
I personally do not think we'll see a L lense with STM anytime soon, for my taste, normal focus is noticably slower with STM than USM (non video). I believe though that Canon will add these feature to all future Cams...
A pity that they did not introduce it with the 5D3 already