In low light, focusing can be quite bad at the short end. The distance scale will land at a random postion if it fails. However, focusing at the long end is very good at low light.
If you've achieve focus at the long end, the distance indicator does not move at all if you zoom out and AF the same subject. This means that during low light, if the AF fails at the short end you can zoom in to focus, switch to MF, then zoom out to take the shot.
This is not a very sharp lens. I think the EFS is sharper. The AF is noisier and slower than EFS 18-55mm. However the 125 end is very VERY useful. You basically don't have to switch lenses at all.
The only reservation I have is this lens is made for 1.6X DSLR. But as long as Canon is still producing EFS lens, I think the 1.6X DSLR is here to stay. I'm looking forward for the Sigma's new 18-50mm F2.8 to arrive.