The big reason is that there's no other game in town. In a world where fast prime lenses are quickly disappearing, this unexpected lens is about your only option aside from the expensive and out of production Olympus 21 f/2. Most people think that f/2.8 is 'ultra-fast' in the wide angle arena. They are wrong!
Another reason that people may not be willing to part with it is how nice it is to manually focus. The AF clutch system is kind of weird, and you are required to throw two switches to change from AF to MF and vice versa. But once you switch to MF mode, you are taken back to a time when lenses were smooth and well damped, and you could make minutely small adjustments effortlessly.
Despite these advantages it is not a lens for everybody. First, it is a large lens. Long and heavy. Second, it is a Sigma. Many people have the Sigma Stigma. I will admit to having it too. I owned a Sigma that broke in the field after just three weeks. One of the internal lens elements came untaped. Yes, you read correctly. The damned thing's lens elements were held in place by double sided sticky tape. I returned it promptly, bought a Canon, and never looked back.
Sigma had quite an awful reputation in the late 1980s and through the 1990s as building bargain basement junk. This may still be true for their budget lenses, but I would not hesitate to recommend this lens to anybody looking for something wide and fast.
As with the vast majority of lenses out there, it is better stopped down than it is wide open. Wide open, it has really good sharpness in the center and is a bit less stunning towards the edges. As you stop it down, it rapidly improves, even beating my Canon 20-35 f/3.5-4.5, which is a better lens than both the 17-40 L and 16-35 L. This is quite a feat. By f/2.4 it is great in the center and has fair edges, and by f/4 it has even sharpness across the frame. It does flare a bit though. Notice the flare in the photograph above (a very tricky situation for that particular lens - bright lights right in front of the lens).
If it was swiped I would buy another in a second. It is invaluable in low light situations where the flash is not allowed or would kill the mood.
