If you really want the 'fisheye look' you will be severely handicapped by any fisheye designed for FF bodies (e.g. the Canon or Sigma 15mm f/2.8 lenses).


The Tokina 11-16mm f/2.8 is a rectilinear lens, not a fisheye (the Tokina 10-17mm f/3.5-4.5 is a fisheye zoom, the only one I think, and it's designed for FF meaning you'd be cropping away much of the fisheye-ness on your 40D).


If you're going to use this as a UWA lens, give careful thought to the fisheye look - is that something you want in all your images, and/or are you willing to do the necessary post-processing (and suffer a big hit in corner sharpness) to remove the distortion?


Else, I'd really recommend getting a rectilinear UWA like the Canon EF-S 10-22mm (or the Tokina 11-16mm, the Sigma 10-20mm, the Tamron 10-24mm, or the recently-announced but not yet available UUWA Sigma 8-16mm).


Let me say it again, unless you really, really, really want the fisheye effect, get a rectilinear UWA. That look goes in and out of fashion like wide lapels and polka-dot ties. Even when it's 'in', it can easily be overused. If you already had a rectilinear UWA and wanted to supplement that with a fisheye, fine. But IMO a rectilinear lens is definitely the place to start.