I just got this lens today. I took it out in the backyard and snapped about 400 photos. My initial impressions:
Build and appearance: Quite nice. This lens is attractive, and feels solid. A big improvement over the 18-135 IS (original).
VF AF: Extremely quick, and very accurate. This lens deserves the designation USM. I would not hesitate to use the 18-135 USM for sports/athletics shooting. More to come.
IQ: I have not done enough real-world testing to comment intelligently. In the center of the frame, at f/5.6 to f/8, it is excellent all the way from 18mm to 135mm. I expect that. Again, much improved over the original version, which was atrocious at the long end, at any aperture. More to come.
LV AF. Very good in both One-Shot and Servo, but in LV Servo, it struggles a bit with low contrast subjects. The 18-55 STM is better with low contrast subjects, but not by much. And that may be my testing methods. And low contrast subject matter really taxes ANY AF system.
Video AF: Quick, smooth, accurate. It lives up to the Canon advertising hype. I only tested with default settings. More to come.
Video noise (sound): I tested this lens against the 18-55 STM. I made no focal length adjustments during the test. In a dead silent room, I can hear what I think is the IS in the background, on both lenses. The sound is minimal and will be easily covered by ANY ambient noise in the environment. With respect to focus noise in video. It is discernible with the 18-55 STM in a dead-silent room; ambient noise will cover it, easily. With the 18-135 USM, focus noise is a bit more pronounced. I am not sure ambient would cover it, but it isn't horrible. More 'real world' work necessary, and coming.
Overall, I am extremely pleased with my new EF-s 18-135mm f/3.5-5.6 IS USM lens. I opted for USM over STM because focus speed/accuracy is more important to me than silent video focusing. I'd rate the focus speed and accuracy of this lens as equivalent/near equivalent to my 85 1.8 and/or 70-200 f/4L IS. It is good. The slight issue with video focus noise is likely NOT an issue.