Simon, Zach’s spot on about resale value, but in some cases the 3rd party suppliers have exploited holes in Nikon’s line so that their lenses are the only economically viable option.
The 3rd party lenses do seem to be a lot more “hit or miss” than Nikkors- it’s rare for Nikon to come out with a dud of a lens, whereas the other suppliers have a more varied field- there are some stellar lenses out there, but there are also a handful of lenses that are just plain bad.
I have two Sigma lenses. Both of them are their EX series which is supposed to be their pro line. One,
(70-200 2.8) is fantastic and I can’t say enough good things about it. The other (28-70 2.8) can be soft. It is not bad enough that I have gotten rid of it, partly because of the resale value as indicated by others. I won’t say I wouldn’t buy them again, but personally, after 2+ years with a DSLR, I now would only by Canon or Nikon (I use Canon), unless I needed something outside their line.