Are you using it for the 60D? I bought mine as part of a kit with the 70D and I chose one reason: video. The STM lens focuses quicker and silently when shooting video compared to USM lenses. Otherwise I would have gone for a USM lens. Reach is great - essentially 7x zoom. Have noticed some vignetting.
18-135mm STM is indeed supposed to be better optically than the older 18-135mm USM. The downside is that it's not particularly cheap unless you get it as a kit option for a camera.
USM? The "old" lens had a micro motor. USM lenses are fast and silent when focusing. Nothing wrong with the old lens though, I've shot some fantastic shots with the mk1 micro motored 18-135.
This is the video I looked at: http://www.youtube.com/watch?v=djOPPQycZ8g And this is one with the 70-200 lens: http://www.youtube.com/watch?v=B-f52_lk4Yk
This is the video I looked at: http://www.youtube.com/watch?v=djOPPQycZ8g And this is one with the 70-200 lens: http://www.youtube.com/watch?v=B-f52_lk4Yk