hehe
at least still cheaper than canon ones
a canon ef15mm non-usm somemore cost $1200+
heard the Sigma EX series quite gd
fisheye is expected to have certain level of distortion
sharpness comparsion hard unless canon started to put a L in its fisheye lens (drooling liao....)
Pretty sharp if you are using a DSLR with focal length multiplier. But then you don't get the full fisheye effect, just a "wide lens with bad barrel distortion" effect. Fun if you are taking mostly people, not good for architecture and stuff.
On a full frame 35mm camera, the edges are pretty soft.