While I don't own that lens specifically, I do have a quick word or two (well, quick for me anyways! LOL!) on Sigma in general.
There is something of a prevailing myth that lenses from third party vendors such as Sigma and Tamron simply aren't as good as those from the camera makers such as Nikon and Canon. Personally, I'm a pretty big Tamron fan myself. While I will openly admit that Tamron lenses (at least the ones I own) aren't as "quite" as those from Canon and that their focus is a bit slower, when it comes to sharpness, I'd put my Tamron 70-300mm up against just about any lens out there. There may be a little difference between the quality of images shot with my Tamron and that of say, an equivalent Canon "L" lens, but the difference isn't really that significant (at least in my ever so humble opinion). If sharpness is your primary concern, than the 3rd party vendors do indeed have some very excellent glass.
I've been reading thru various lab tests comparing the sharpness of the third party lenses compared with that of the camera makers and really, when it comes to image quality, the difference isn't usually all that significant. The biggest difference really seems to be in "the build" of the lens and not the optics...and to me, the optics are the important thing. In fact, in the February 2008 issue of a national photography magazine (which will go unnamed), there is an article titled "Lens Special: Behind The Glass" which addresses 10 of the most asked questions about camera lenses and question #3 is "Are lenses by the independent makers as good as those by the camera makers?" and one of the things they say is "Over the years we've seen many examples where the lenses from the top three independents - Sigma, Tamron, Tokina - perform on a par with the equivalent lenses from the camera makers, and occasionally even better".
I will say that I haven't really been as happy with my Sigma 17-55mm as I could be, but it also has to be said that I bought that lens very used off Ebay and that while it was a cheap lens to begin with, I only payed $35 for it so I can't really complain in any case. It does ok, but it certainly could be sharper. My old Sigma 70-210 for my old Canon FTb however was always an excellent lens! It shows some rather serious barrel distortions of course, but that was simply typical of lenses of that range in that day and age. It was (is) however a -VERY- sharp lens.
It seems like you're doing your homework on this. I'd have to say trust the reviews...if it sounds like what you're looking for, I say go for it. Sigma has some good stuff out there to begin with and I've heard some good things about the 105 macro. If your concern is that this lens is a "Sigma" and not an actual Nikon, I'd say don't sweat it and give the lens a shot.
Anyways, that's just my $.47 worth...keep the change