I have that Pentax 28-50mm and would use it more often if it were a bit faster. It's f3.5 at the 28mm end which isn't so bad, but f4.5 at the 50mm end gives a dim viewfinder. It's fine in good sunlight but not so wonderful in city streets on a dull day.
Mine is a fungus survivor, I had to take every element out, clean it, and rebuild it. Despite this and the inevitable dust it's decently sharp, only slightly behind my SMC P 28mm f3.5. However, I'd rather carry the 28mm and my 50/1.4. I keep meaning to strip the zoom down again and see if I can get it back together with less dust (and without the speck of fibre I noticed just as I tightened the last bit up).
My main complaint with zooms is the size and weight required for comparable performance and aperture to a prime. I also have the Pentax-M 35-70mm f2.8-3.5 which while a good lens weighs a ton (you really need a winder to balance it on an M series body). Meanwhile the 35mm f2.8 from the same range is only slightly longer than the 50mm.