Great butterfly portrait
Here is my experience with a sigma 150 macro, sounds similar to what you describe for the 180, I started using it with a D300, AF was useless and if I tried I just got frustrated.
Got the D800 and the AF on the sigma 150 became useful, rarely do I have to manual focus, but years of practice do support the recommendation of using the distance limiting switch, and I set the focus in the ball park distance by hand, then AF, that is when shooting by hand.
On a tripod with subjects really close, I'll focus by hand