The lens has been improved in a number of ways. The most significant change is a redesign of the optical construction which now features a UD element - something that we usually only see in Canon L grade lenses.
The AF speed has been significantly improved to a very decent level without being as fast as ring-type USM lenses. Thanks to a micro-USM drive the noise level is very low. The lens does not feature FTM (full-time manual focusing in AF mode) though. Due to the rather slow max. aperture the EOS 350D had sometimes a hard time to find a really accurate focus at 300mm - this is not specific to this lens though.
One key selling factor for this lens is naturally the Image Stabilizer (IS). Camera motion is detected by 2 gyro sensors which measure the angle and speed of the shake. This information is used to shift a lens group off the optical axis (basically a forced decentering) to counteract this motion. The result is a significant improvement of handholdibility under extreme conditions. Canon improved the efficiency of the mechanism to an equivalent of 3 f-stops (up from 2 f-stops).
The IS now offers 2 different modes (instead of just one):

• mode (1) for static scenes where the IS corrects shake both vertically as well as horizontally
• mode (2) for action photography (object tracking) where only vertical shake is corrected

The build quality of the lens is about average and maybe a tad worse compared to its predecessor. Neither the focus nor the zoom control ring is especially damped but at least pretty smooth. The various switches feel a little cheap. As you can see in the product shot above the lens extends significantly during zooming (typical in the lens class) and the front element rotates during focusing operations which means that using a polarizer is a little cumbersome at best. Another new feature is a transport lock that prevents zoom creeping - unfortunately only at the 70mm setting.

The minimal focus distance is 1.5m resulting in a max. object magnification of 1:4. The optical design is made of 15 elements in 10 groups with one UD element. The lens features 8 aperture blades. With a size of 77x142mm and weight of 630g it remains a quite compact lens qualifying it as a good walk-around lens.