By the way, which camera body are you using? If you are using a full frame camera (5D, 5D II, 1Ds, 1Ds II, 1Ds III) or 1.3 crop body (1D, 1D II, 1D III, 1D IV) then the 24-105mm f/4 is an excelent choice, the 24-70mm f/2.8 is another fantastic lens. But if you are using a 1.6 crop (Rebel series, 40D, 50D, 60D, 7D, ext) body the 17-55mm f/2.8 is considered the defecto for 1.6 crop bodies, sharper, faster aperture than the 24-105mm and has IS which the 24-70mm lacks. And for most people (myself included) a nicer focal length range.


Hope that helps,


John.