Originally posted by panoguy Sigma is showing some weird Magenta/Green patchiness, while the K-5 is pretty much smooth, tonally. I think this shows both a flaw in the Foveon color response to bright neutrals, and the advantage of blurring (AA filter) the image a bit at first, to then try and recover it with sharpening later. Subtle tones across larger areas are better preserved in the K-5, but tiny bits of color are picked up by the DP2M (though those stones are not sprayed with magenta and green!).
Getting rid of the green and magenta patches is easy, just use the base ISO or expose the shot a bit more. I've had those on some less lit surfaces when slightly underexposing in, say, ISO 200 or 400. Lower the ISO to 100 and/or give the darker areas some more light, and the blotches are gone, and you're left with smooth and detailed surfaces. That's a known Foveon issue, it's not a high-ISO camera. I hardly ever use anything but ISO 100 and 200 in my Sigma DSLR.
Shooting with a Sigma/Foveon camera is the closest you can get to shooting slide film, both in good and bad. Your range is rather limited but, when working within that range, the results are sweet. I've grown to like the Foveon look in all its quirkiness, after I learned to embrace it as it is, and appreciate it for what it is, and not trying to agonize or fight against what it isn't. It's nowhere near perfect in any way, but at its best it's very nice. A pure enthusiasts' choice, certainly not everyone's cup of tea.
Oh and I agree about the ACR thing, why bother, it's apparently made for Bayer sensors only. I've never used anything but SPP and Aperture for my X3F files. Maybe an occasional tweak in onOne Perfect PhotoSuite, too, once the first conversion has been done in SPP.
But in general, to get the most out of the Foveon sensor, always shoot RAW only, and keep the files far away from anything Adobe.