Two pictures from the harbor area in Oslo (Aker Brygge). One is taken with a Sigma 10-20mm and the other with the Sigma 4.5mm.
The fish-eye can create some cool effects, but there are several trade-offs. The 4.5mm use just 1/3 of the CMOS real-estate and it there is somewhat brutal process to unfold the picture from a circle to a square format. This could reduce the sharpness significantly. Below you see the results. Let me know what you think.
Aker Brygge - Sigma 10-20mm by janokiese, on Flickr
IMAGE LINK: http://www.flickr.com/photos/jankiese/6667959963/
Aker Brygge Sigma 4.5mm by janokiese, on Flickr