Being an ultra-wide-angle, it will be interesting to see who this lens is aimed at. I might be in the market for it if it is priced sensibly and outperforms the Canon 14mm.
I just hope that this lens is not prone to complex, moustachioed distortion, as is the case with several of their lenses.
My use for such a lens is mainly architecture. I haven't tried the Canon 14mm, but, its mtf/resolution figures show that (especially stopped down from f/4) it is quite sharp across the entire plane. Also, it is remowned for it's very low distortion given how wide the lens is.