I've only played around with one at a large retailer in Tokyo. It is significantly larger and heavier than its little brothers, namely the "Contemporary" version and the Tamron equivalent.
Sometime ago, the Japanese web magazine "Digital Camera Watch" posted a long interview to the designers of these two Sigma zoom lenses. They said that you can zoom in and out by pushing and pulling the front barrel directly, even though there is a dedicated zoom ring. This way you can change the focal length much faster. The mechanism is designed to withstand such a wild operation.
On the other hand, they asked the users to refrain from doing that with the "Contemporary" version and operate only the zoom ring to change the focal length.
