I have had my 10-17 for almost 5 years now. As others have posted its a fisheye across the entire range, however at the widest - 10mm it appears to be a true fisheye. As you progress through out the range to the narrowest end, the fisheye effect appears very subdued. It is still there, but you really need to look.
Another approach to judge is the Angle of View (AoV). At 10mm its 180 degrees corner to corner, while at 17mm its 100 degrees. The DA 12-24 has a an AoV that starts at 100 degrees (12mm) and goes to 60 degrees (24mm). Based on these two lenses AoV wise they complement each other perfectly, however based on focal length there appears to be a substantial overlap. This is due to one being a fisheye while the other a normal or rectilinear lens.
It is indeed a specialty lens, however it is incredibly fun to use. I doubt that you would want to use it daily, however there are applications that it just excels at. Getting close to very large objects, and then grabbing a wide shot where you have things in motion and stitching will just not work.
The fisheye effect is very pronounced when you shoot objects with very straight lines and square corners. Scenes with lots of natural curves - trees, mountains and the like the fisheye effect can be very gentle. You, as the photographer, are able to determine where you want to place the "bend" in the image by ever so slightly tipping the camera up or down. You can have a very straight horizon also.
Just one word of caution - when using it, check the frame for where your toes are - as its easy to get your feet in the bottom edge of the frame.