Hi Graeme,
A few months back I was shopping around for a Nikkor AF-D 200mm f/4.0 IF-ED Micro lens and found a used one only 5 miles from my home without papers or box.
After I tested the lens in comparison to a brand new Sigma APO Macro 150mm. f/2.8 EX DG HSM that I tested before, I came to the conclusion that the Nikkor is very good and sometimes better than the Sigma, but also very expensive, even if it's a second hand item. (The guy wanted 950 Euros for it and still hasn't sold it....

)
So, that's why I bought the Sigma (without OS because I think that's pretty useless on a macro lens) for 649 Euros (with OS it costs 839 Euros) and haven't been disappointed by it yet.
The Sigma will deliver at least 90% of what the Nikkor can and that's good enough for me.
Image quality is really excellent, AF is spot on, MF is very controllable, it weighs less and the only difference is that it's 50 mm shorter.
So, if you really want to buy the Nikkor, you should do it, but only if you plan to sell it again in the future without loosing money on it.
If not, go and find a Sigma new or used and have fun with it!
PS: The 180 mm Sigma is not as good as the 150 mm!