Executive Summary. The 15mm fisheye allows significantly greater angle-of-view than a 14mm rectilinear. The angle is so wide that flash coverage requires special attention. With computer software, the fisheye view can be changed to rectilinear, if desired.
Like most DSLRs, the D30 makes life difficult for wide-angle users. The 1.6 multiplier turns the attractive 24mm into a 38mm. I seriously considered the following lenses:

Canon EF 16-35mm f/2.8L zoom
Canon EF 14mm f/2.8L
Sigma 14mm f/2.8

The 16-35 is heavy, large and expensive -- and I just want the 16mm part. The Canon 14mm is heavy, large, and astonishingly expensive. The Sigma 14mm is heavy, large, and subject to variable build quality; I did like the price, and was close to buying.
However...

After viewing some samples of the Canon EF 15mm f/2.8 fisheye, before and after processing by Panorama Tools, I began to see another way out. So I bought one (shameless plug for Delta International here).

Technical recap of wide-angle lenses:

A rectilinear lens will preserve the straightness of lines throughout the frame, but objects at the edge become stretched, losing their proportion. These lenses are desirable for architecture. For normal appearances, keep people away from the edges.
An angular fisheye preserves the angle between two lines, but lines become increasingly curved at the edge of the frame. Subjects with recognized proportions, like people, will look best with this lens. For normal results, keep straight lines away from the edges.

	         |  14mm rect  14mm rect  15mm fish  15mm fish  15mm fish
	axis     | full frame  D30 frame full frame  D30 frame  D30 frame
	         |                                             linearized
	---------+-----------  --------- ----------  --------- ----------
	diagonal |        114         88        180        113        104
	width    |        104         78        150         94         94
	height   |         81         56        100         62         62 ¤

	¤ Confirmed at my optical bench (desk, ruler, pencil, paper)  :-)

The table shows 14mm rectilinear beaten by the 15mm (in fisheye or rectilinear mode). In rectilinear use, the fisheye lens requires cropping: this reduces original pixel data and reduces resolution. These losses are minor, and occur in the corner only.

Key observation: The Canon EF 15mm fisheye, used with a D30, linearized, and cropped to full frame, is equivalent to a 10.6mm rectilinear lens!

Flash issues arise with this extreme coverage. From the D30 manual: "Covers field equivalent to a focal length of 18mm". While ambiguous, it sounds impressive... or maybe not. Being charitable, I get 100° diagonal coverage, which is not quite enough for linearized pics, and entirely inadequate for fisheye. I tested coverage of the built-in flash and the Canon 550EX external flash.

In summary, the D30 and fisheye were made for each other. Versus the 14mm rectilinear, you get the following: better price, super-wide coverage, lower weight, smaller size, no light fall-off, sharper corners.

Links:

Panorama Tools by Helmut Dersch is open-source software to perform sophisticated geometric transformations on your images.
AcaPixus has good samples of de-fished images.

Photo gallery follows...