more on the zenitar may be useful, if on a budget:
while the 12-24 certainly commends respect, the zenitar is incredibly small and light, while well built. i absolutely love it. optics are very good in my oppinion, there is some CA in extreme conditions, never bothered to correct them, never bothered me in prints (yet). being a fisheye, it will be visibly distorted even on an aps-c sensor, it is easy to correct in software (even batch), however one word of caution: when de-fishing, some of the image (mainly corners) will have to be cropped, this means that what you see in the viewfinder will not be what you get at the end, and you have to keep this in mind and plan for it when framing, if using it in place of a rectilinear wideangle.
edit: the fov of this lens, after defishing (so in "linear wideangle" terms), would be something around the fov of a 13mm or 14mm. that is to say, this will be wider than you expect from the 16-45, make no mistake, and a hell of a lot wider than the 18mm on the kit lens.
so, in short:
pros:
-price (about 200 usd in US, if not less)
-optical quality
-built quality
-size and weight (with an "!")
-looks sexy on the k20d
-it transforms instantly into a 180 degree fisheye on your backup film body, incredibly fun
cons:
-manual focus (or is that a feature? who needs af on an ultrawide?)
-not pentax, so no smc coating (there is flare in extreme conditions, not terrible, but the kit lens is able to arrogantly show it what a pentax lens thinks about flare
)
-k compatible (or M, not A), so no aperture control from the camera, and only stopped down metering on dslrs. i ended up using only such lenses lately, so i am kind of used to it. ymmv
-the hood is designed for use on 35mm film, so there is virtually... no hood (180 degrees fisheye)
-no filters (i actually need to use a polarizer filter on it, despite what some people think, polarizers are not only for skyes). can be worked around, i hear (but only for aps-c)
i think that should sum it up