From DPReview Conclusion:
The Sigma 12-24mm F4 offers impressive optical performance at the short end of its focal range, with excellent AF performance. Unfortunately, central sharpness at all focal lengths, and overall sharpness at the tele end suffers relative to its best peers. Severe focus shift at longer focal lengths require you to focus the lens at the shooting aperture, which can be cumbersome, and will also come at a cost to corner sharpness. Other optical properties, like CA, vignetting, and distortion are extremely well-controlled if not class-leading. If you’re looking for a wide-angle zoom with very nice performance from 12mm-16mm then this lens is an exceptional value.