I now have a rare 17mm f/4 Pentax SMC Fish-Eye lens, complete with its original case! (You can tell it's old because fisheye is hyphenated.) It definitely takes one beyond the 18-135's widest views, and with a twist thanks to the interesting distortion though on aps-c sensors the distortion is relatively low. This is a late-70s lens¹ from the original Pentax bayonet-mount cameras, before the M series (and A, F, FA, and finally modern DA series) so manual focus and aperture/metering only. Among other interesting things this lens focuses very closely at just under 8 inches, which leads to decent bokeh and great composition opportunities. It works with film bodies too!
First images compared to 18mm on the DA 18-135 show the reach difference especially to the right and top on this test shot. Exposure is a bit brighter too, and the coatings result in a loss of contrast which loses details especially in the outside shed. The fishiness is also quite clear here along the outer edges of the frame. Still it will be great for outdoor vistas, if not so much for indoors like this :^)
This lens could crowd the 24mm a bit, we will see. The 8mm Rokinon will definitely suffer with the 17's arrival, so that one may go before it sees much use at all.
¹ It seems I have the 2nd edition of the lens, but it's still a classic from the earliest K-mount days!