Sharpness, lightweight, small, IS, you have 420mm and 600mm using canon extenders.
slow AF, noisy IS
The 300 F4 is an excelent lens, the image quality is superb, even at F4, great for birds/wildlife, sports, portrait and using an extention tube very good for macro. You can use with 1,4x and 2x extender (with 1D body you don't loose AF)
the only two bad thing with the lens is de slow AF and is not weather sealing