I'll keep it short since I just bought it. It's more my first impressions than an in-depth review.
Context: I used to own the non-STM version of this lens so I can compare both. My camera body is a 60D.
1- Build quality (zoom ring / barrel): Surprisingly smooth when zooming, really no play when extending.
2- Auto focus: Pretty quick, noticeably quicker than the previous version. I guess it takes a little to get used to it as it's almost too quiet.
3- Manual focus: Although it's nice to have full time manual focus, the fact that the ring can turn indefinitely is disconcerting. Never too sure if there's more to focus or if you already reached the end of the range... Weird. Another characteristic to get used to I guess. Doesn't spin as freely as the previous version (which is good; makes it feel better built) but I'd like it to be a little more dampened (especially since it's STM, there's nothing to feel as it's not mechanically linked to the lens).
4- Image quality: Surprisingly good. I haven't had time to do a full range of testing, but so far, it's very sharp. Corners and centre, doesn't matter, it's clean.
5- IS: Super effective. Give it 1/2 to 1 sec to get up to speed. Once active, the effect in the viewfinder makes you think you're a hero at hand holding a telezoom!
6- Plastic mount: I couldn't care less... It mounts securely and stays super stable on the body.
7- Balance on a 60D body: Feels great, well balanced. The 60D has a decent grip for the right hand and with this lens, the grip on the left hand feels balanced to the other hand.
That's what I have in mind for now. I'll update if I discover anything else. If there's something I haven't covered you're interested in, let me know!