I've had a couple of the sigmas over the years (mostly bought in the spring when I start thinking wouldn't macro be great), the reviews I've read seem to think the sigma is slightly better than the Nikon. The price of the sigma new at £319 at present is pretty good, you can pick them up for less. I can't remember what I sold the two lenses for but it wasn't a big loss from new. The only reason I sold them was because I just wasn't using them, I think that's the reason many sell them for rather than them being rubbish. The AF is noticeably slower but then I often use a 70-200 f2.8 and for macro it's easier to use manual focus set at 1:1 ratio and focus by moving in and out or with the focus ring.
Part of letting the macro go was because I've been trying to minimise the kit I have to what I actually use often enough to justify keeping it. I've managed to get my lenses down to 4, maybe even 3 as I'm finding I'm just not using the 20mm f1.8 as much as I thought. The 24-120 f4 is doing majority of my landscape stuff.