Quality-control on Sigma's teleprimes

Hi guys,

I'm eager to get a quality 400mm-or-more-telelens for nature photography.
The 500mm/4 and 400mm/2.8 af-i/af-s are somewhat too big, heavy and expensive (although second hand af-i are well priced) and the 200-400mm/4 is not all that bad in size/weight, but goes for more that 5500 euro.

But... new hope rises. The sigma 500mm f4.5 APO EX HSM may not have the ultra fast AF, but the image quality comes very close to his Nikkor counterpart (according to some reviews) and its lighter, smaller and best of all: priced at 3500 euro.

The only thing i'm worrying about (as a enthusiastic nikonian) is... the brand Sigma.
Without any disrespect, but I heard some rather unpleasant quality-control stories and I don't even wanna get in that kind of trouble.

So... how about QC on those teleprimes, is that any better then their 'regular' lenses?