Although I haven't used one... I'm not convinced about the new 70-300 L. I have the 70-300 f4-5.6 IS non L and in terms of image quality it was always reckoned to be up there with L series lens (although not the same build quality) - have a look at
www.slrgear.com and you'll see that at some focal lengths/apertures the (much) cheaper lens is in fact sharper than the new one
However these days I find myself very rarely using it - the (comparatively) slow maximum aperture is a major drawback for me so I reach for my 70-200 f2.8L every time - its awesome. I did have a Sigma 120-300 f2.8 which produced great images but it was sooooo heavy that I sold it and bought the 70-200 and haven't regretted it yet