i just got a Rokinon (Samyang) 14mm f/2.8. like the 8mm fisheye, it's all manual, no AF/AE, it doesn't even have electrical contacts on the lens mounting ring. but it's surprisingly sharp, with a lot less distortion than the 8mm, and will give you an 89-degree field of view on your 7D - that's from the horizon to straight up overhead. plenty for shooting the Milky Way, etc. it's probably a more useful focal length than the 8mm, too.
i did some informal testing comparing it to the Canon 24-105L for corner sharpness, etc. the Canon lens is brighter, but the Rokinon is nearly as sharp in the corners as the Canon at f/4, and at f/8, it actually looks a little sharper! that's on my 5D Mk II - on your 7D, you won't have any issues at all with the corners. for $339, it's a bargain.
Out where the rivers like to run, I stand alone, and take back something worth remembering...
~ Three Dog Night