David Scott , Jul 29, 2009; 02:04 p.m.
I have and like that lens, but it's not the most useful focal lengths on my K10D. I prefer this lens on my MZ-7 (film) camera, where it really is a moderate wide to moderate tele.
I find this lens to be quite sharp, with nice contrast and lovely colour. It has the optical look of a true Pentax SMC lens -- I have no complaints about it optically. It's probably the sharpest lens I own after my FA 50/1.4.
I also really like the macro ability of the lens - though it's not automatic, you need to manually rotate the zoom ring into the macro range.
Another advantage -- it's a 49mm filter ring, so I can use my nice selection of 49mm filters.
Biggest disadvantage is that it's a relatively slow (f/stop) lens. You will especially notice the difference from your Sigma 2.8. The Pentax 35-70 is definitely best suited to daylight exteriors.