After having exposure issues with Tamron's 90mm macro lens, I swore I wouldn't buy anything other than Nikon lens; however, at $469.00 for a brand new Sigma 180mm macro I had to choose Sigma. After a couple of days of playing with the lens on the D100 I think it performs flawlessly. For my budget it was a wise chose. The autofocus, though somewhat slow, is so quiet you can barely hear it. Very impressive for <$500.00!
Many of the bad rumours about Hotbuys are indeed true. I've had one so-so and one very negative experience with this outfit (see Metal vs. Plastics lens mounts in this forum). It's not just one rogue Hotbuys sales rep, but at least three that I've dealt with -- it's not worth the hassle to deal with them. I just ordered and received without any hassle two Sigma EX lenses from Tristate Camera and would order from them again.