i dont have any hands on experiance of this lens but ive read loads about it as ive been debating the perchase of one for ages,
it seems to be a very good lens for the price, although i wouldnt agree with that now as the price is around £1100 , when it was marked at £700 8 months ago i would say its a winner,
its now been "replaced" with a 150-500 OS but they dont seem to be getting as good reviews as the 50-500, i think the OS is quite important with a "walk about lens" capable of going to 500mm because you dont always have a tripod, especially with a maximum aperture of f6.3 and to get best results you would need to stop it down to around f8 so when shooting handheld for best results you need a shutter speed of 1/500 at f8 which can be a dificulty especially when its not best to shoot in bright sunlight becasue the light is too harsh,
but there has to be a draw back somewhere with such a cheap long lens.