Originally posted by Adam Sigma lenses are very good. Not all of them are amazing, but the high-end ones certainly are. And what's funny is that Sigma has a larger available lens selection than Pentax do themselves.
I would say for (slow) walkaround lenses, stick to Pentax. The Pentax 18-135mm and
50-300mm are both excellent lenses.
plus one to this Adam, sigma make some excellent lenses (particularly the fast primes and 2.8 zooms.) but i'm less impressed with their low end like the 18-125 (or the 55-200 for that matter)
thankfully they can just adapt the mount to the lenses they build for canikon giving us a selection of product Pentax doesn't make right now