The first surprise of this lens is that it doesn't feature a whisper-quiet hypersonic Sigma motor.
Instead, you get a conventional focus ring by the front element, which doesn't enable manual override in Autofocus mode. It's detached from the front though, which doesn't rotate during focusing. Even without ultrasonics, it's quick and quiet.
The second surprise is the image quality. From its macro range of 0.2m to infinity, our test images were pin-sharp, with superb colour and contrast.
Vignetting and barrel distortion are slight at 18mm and there's no pincushion at 50mm. Chromatic aberration, ghosting and flare were absent. It's a reasonable price, and available in Canon and Nikon mounts, as well as Sony/Minolta, Pentax, Sigma and Four-Thirds.