Never used one, but what I have read in reviews is that they are fairly sharp wide open from 50-200mm, but go somewhat soft beyond that. In order to get sharper at the longer focal lengths (200-500mm), they need to be stopped down a stop or two. Unfortunately, since this is a variable aperture lens, at 500mm you are already up to f/6.3 wide open, and if it needs stopped down to f/8 or f/11 for sharpness, you are going to need either a lot of light or very good high ISO performance (or both). This likely isn't a lens you are going to be shooting birds with at 500mm on a cloudy day. I have a 500 f/4, and on a cloudy day or lower light conditions, even I have to bump the ISO a few stops to get decent results.