It is super-sharp, never fuzzy in the corners like Nikon fisheyes can be on a bad day.
The Canon 15mm fisheye fills the full frame with curviness and waviness, seeing 180º from corner to corner. Groovy!
The 15mm fisheye is one of the oldest lenses in Canon's catalog. It is unchanged since its introduction as one of the original lenses introduced with the EOS system in 1987.