Contrary to some rumour circling around - I personally find this lens to be extremely sharp - for what it is I'd hardly expect anything more. besides it is also covering the full frame and pretty good performer on film - so all in all - a very neat lens that can come in handy ! Often if I go with one lens - I pick zoom for convenience, but then I ALWAYS get this - so for me one lenses usually means 1 + 40mm XS :P
PS: Below two sharpness samples - make sure you don't get anything much worse than that, otherwise you copy could be faulty ( this is where rumour mentioned comes from probably - people get faulty lens and assume this is how they are ).
f2.8
f5.6