I have used the 55-250 STM as a test for concerts and animal shooting. I haven't measured it, but to me the STM seems to work just fine, although it feels a tad slower (?) than USM (still significantly quicker than non-USM though). The one thing to keep in mind is that the focus on these STM lenses is electronic, so you can't "full-time manual focus" without power to the camera.
I don't want to be a lens snob, but as a still shooter with a bag full of USM glass, I only see STM as an improvement over the non-USM lenses I left behind years ago. I have yet to hear of a single thing it does better than USM for stills.
So STM, for me, says 'for video', and as such, won't be getting any of my money.
As a stills shooter I prefer USM. But as you said, STM is a nice improvement over the stiff, noisy AF motor it replaces. So it's good when viewed in that respect, but it is no replacement for USM (and probably why we haven't seen an "L" lens with STM at this point).
But, even with that said, I think the only times you'll notice a difference are when 1) you are tracking a moving subject in Servo (less snappy although still quick enough in most situations) and 2) when manual focusing, unless you have an electronic USM lens like the 85 1.2.
I wouldn't recommend any of the STM lenses for serious (stills) work but for everyday/travel use, I gotta say they're quite nice...pleasantly surprised. Good if you don't want to risk more expensive stuff on a trip, etc.