Which FISHEYE Lens??

I am in the market for a fisheye lens for my 40D. Went to the local camera shop today to try some out and left feeling more undecided than when I entered.

I initially interested in the Canon EF 15mm fisheye but after trying it out on my body am disappointed with the loss of the 'fisheye' effect on a APS sensor. They did not have any of the other options for demo.

Does any one have any feedback or samples from the sigma, tamron or others? Suggestions of other lenses to consider? The new Canon fisheye is appealing however I would like to keep the price under $700 for such a specialty lens.