I have the f/4 IS version and use extenders with it all the time. I use mine inside a bunch and just have to bump the ISO up another notch, but if your not shooting fast subjects indoors with it, the IS is awesome in that lens. I use mine outside for landscapes, birds when needed, and kids sports. The AF is as fast if not faster than the f/2.8 version. I think the f/4 IS version is sharper than the f/2.8 L non-IS.