What kind of camera? Which lens manufacturer? If it's Canon on a 10D or D Rebel, then it's closer to 24mm when figuring in the sensor chip size. Still respectable though.
Remember, that any lens of this nature, though not a true "Fish Eye" lens, is going to give you lots of barrel distortion around the edges. The real true perspective will be a small area in the center of the photo and then the distortion gradually increases out from there. As long as it's not too noticable, it should work out alright.
Aaron and I think a couple of other have some experience with ultra wideangle lenses. they can give you a better jog on this. I've been using a .45x WA extender from my Olympus E-10 mounted on the end of a Sigma 28-90mm on the Canon 10D. I get a little trouble focusing this jerry rig, but it doesn't do too awfully bad.