Originally posted by Spock I also have the FA28-105 F3.2-4.5 and find it to be an excellent lens for the money. I have not owned that particular Sigma lens but I have owned a few Sigma lenses and personally I would always choose a Pentax lens over an equivalent Sigma.
If you were tossing up between a 'pro'-level sigma lens, say with constant 2.8 max aperture then I'd say go for the Sigma - but in this case the Sigma has little real advantage on paper over the Pentax in terms of aperture (1/3 stop faster at 28, 1/2 stop faster at 105). On the other hand the Pentax has a superior coating and is almost certainly better built.
i second that. i had the FA 28 - 105 as well, quite a nice little lens, as everyone said it has nice colour rendition and decent IQ for a better-than-kit zoom lenses.
That being said, I can't really say I felt much for it since it was quite plasticky and light, and my copy suffers from zoom creep. Though these are really personal, minor issues, it is because of these reasons that I didn't take to the lens. I went around and got a tokina ATX (which I sold) and finally settled on a sigma 24 - 70 /2.8.
I guess my point is, if its really a choice between the sigma and Pentax lens you selected, the pentax is a better choice - but if you can afford it (or am willing to wait a bit and save towards getting a better lens) do consider some of the 3rd party "pro" grade lenses in this range, they are worth it.