It's a great general-purpose lens. It's not as solid as the 24-70 or 28-70, but better than the 28-135. The sharpness difference is minimal on a digital camera. I find that the difference in depth of field between f/2.8 and f/4 at shorter focal lengths is minimal. Then again, I haven't had very good experiences with the 24-70 at shorter focal lengths (poor focusing accuracy).
I've only had one incident of the infamous flare problem so far. Interestingly, these images were taken within seconds of each other.