I've owned this lens for several months now and used it a great deal.
For macro shots, it just leaves the opposition in its wake. The shots are pin sharp and being 150mm, I can stay at a reasonable distance from insects etc. without scaring them away. Also, it allows you to use a standard flash, leaving room for the flash illumination to reach the subject. The AF tends to hunt sometimes, but 90% of the time it's fast. I wish it had f/32.
Now for the bad. I'm sorry to rain on everyone else's parade, but I have to question build quality. When I started using it as a standard telephoto lens, I discovered it front-focusses noticeably. When I contacted Sigma (in Australia), they claimed the lens couldn't be calibrated at all. I've had appalling service here from Sigma and it certainly makes me think twice before ever purchasing another product from them. It's a shame because this lens has such potential. I may just sell it and try again though.
I have to take two points off for front focus and one for service.