Specifications

Here is a full circular fisheye sample photo.

You are looking at everything in front of the plane that runs across the front of the lens. I positioned the camera directly above the windshield frame on our '92 MasterCraft ProStar 190 ski boat and instructed my brother to drive us into the sunset. You see everything directly above, below and to the sides of the camera as well as everything in front of the camera. Amazing. 
 
Of course, using the Canon EF 8-15mm f/4 L USM Fisheye Lens on smaller sensor format cameras produces differing results. More specifically, these cameras do not allow the full circular fisheye option. Following is a Canon-provided diagram that illustrates the image circle coverage over the focal length range.

Though the diagonal AOV does not change much as the focal length is increased on a full frame DSLR, the size of the details in the image are noticeably enlarged and the image circle becomes cropped on the sides until the point a full frame fisheye image is reached. APS-H format (1.3x) DSLRs (such as the Canon EOS 1D Mark IV) reach full frame fisheye coverage at about 12mm and APS-C (1.6x) bodies reach full frame fisheye coverage at about 10mm. 
 
When used on APS-H and APS-C format bodies, the Canon EF 8-15mm f/4 L USM Fisheye Lens delivers partial circular fisheye results (black rounded corners) that are probably not going to be desired much of the time. The 8-15 delivers the same results on a full frame body when not used at the full extents of the focal length range. 
 
Unless you are planning to further process your images (cropping or removing distortion), you probably are going to want to avoid this physical vignetting in your images. To assist in avoiding the black corners, Canon provides "H" and "C" markings on the side of the lens for APS-H and APS-C DSLR owners respectively to know their full frame coverage focal length limitations. Note that when focusing close, a slightly longer focal length may need to be selected to avoid the dark corners. You can see these marking near the zoom ring in the images below.

Full frame users can simply use the two extents of the zoom range (8mm and 15mm), though 14mm will be slightly more appropriate than 15mm when focusing at infinity. APS-C users have a focal length range limiter switch (shown below) to physically prevent them from zooming to corner-darkening focal lengths.

APS-H and APS-C users will have some beyond-full frame fisheye focal length range available to use. 
 
So, the Canon EF 8-15mm f/4 L USM Fisheye Lens is a fisheye lens for everyone. Having full frame fisheye coverage for all sensor format DSLRs in a single lens is really useful - especially for those with various sensor format DSLRs in their kits. And those with full frame bodies have the huge circular fisheye bonus. The Canon 8-15 is the first zoom fisheye lens available for a DSLR camera to offer both circular and full frame image circles. 
 
In the Canon EF 8-15mm f/4 L USM Fisheye Lens owner's manual, Canon at one point refers to the "telephoto end" of the focal length range. I laughed. The longest end of this focal length range, 15mm fisheye, is equal to the widest AOV available in a Canon lens prior to the introduction of the 8-15 - the Canon EF 15mm f/2.8 Fisheye Lens. 
 
Below is an example of what the Canon EF 8-15mm f/4 L USM Fisheye Lens' focal length range looks like. The first and last mouseover rows are from a full frame body while the middle row is from an APS-C body. 
 
In the first row, the comparison focal lengths in parenthesis are from the original 15mm fisheye lens and rectilinear results from the Canon EF 14mm f/2.8 L II USM Lens and the Canon EF 24mm f/1.4 L II USM Lens. The parenthesized focal lengths in the second row are from theCanon EF-S 10-22mm f/3.5-4.5 USM Lens.