Great sized lens.
Very Fast.
Sharp images.
Great Color.

My only complaint is lack of weather proofing.
I would pay a little more for this Sigma!

Quality Control:
My current copy is great!
My first copy sent from Amazon had several small bubbles in the glass which could be seen when looking from the back to the front.
This did not show up in the images, but for a 'pro' level lens, I want clear glass!

I recently purchased this lens for my Nikon D200 on Amazon.com
I had since this time been shooting primarily with a Nikkor 35-105 3.5. I really enjoy this lens for its versatility, but I wanted something with a closer minimal focal distance, which was a little faster than 3.5.
I found that the comparable Nikon glass was more than I wanted to spend.
After researching various lenses, I decided to go with this one and I am happy that I did.

The lens is big, but to me this is a plus, I like a lens that has some body to it that I can hold onto.
Some people say the lens is heavy, but compared to other Nikkor lenses in my bag this lens is not heavy at all. Although the lens is not built like a tank, it does feel well put together. My only complaint is that the lens is not weather proof. For their 'pro' line, I wish Sigma would make this happen.

The auto focus is fast, I have not experienced any hunting. Some people complain about lack of HSM, but I don't find noise from this lens objectionable.

As for image quality, I CANT tell a difference between this lens and my Nikon glass. I have seen countless technical reviews which find some differences between this lens and Nikon or Canon glass, but I have looked at shots from my Nikkor lens and this lens and am hard pressed to find a difference.

Here are some sample shots taken with this great lens
http://www.pbase.com/spadephotoworks/golden_gate_park
*All except the second to last shot.

Customer Service

Friendly customer service.
I called to complain about the bubbles and they recommended I exchange it through Amazon, but they said they would happily send me a replacement if I had problems with Amazon.