The biggest problem with Foveon sensors is that they get noisy at relatively modest ISOs; which is another way that my D300 kicks butt on both Sigma DSLRs and film.
Hmmm... that's not what I've found. I picked up a Pentax K20D, a 14MP camera that will do as good or better than a D300, because I wanted something 'better' than my Sigma SD14 in terms of resolution and camera functionality. In short, I wanted a dSLR that would not have the limitations of the SD14.
The K20D is a pretty amazing camera, and because it will also take M42 lenses, I can shoot the same scene with the same lens with both the SD14 and the K20D. What have I found?
I've found that the SD14 is VERY competitive with the K20D in terms of actual resolved detail and image quality. In fact, I'd give the nod to the SD14. Where the K20D excels is in better/faster AF, in-camera image stabilization (handheld shots with a 300mm lens at low shutter speeds are very sharp), and multiple shot capabilities (for bracketing and capturing action). With the latest firmware the SD14 is excellent at ISO 200 and below, very good at ISO 400, and acceptable at ISO 800, while the K20D is excellent at ISO 400 and below, very good at ISO 800, and acceptable at ISO 1600. In other words, you get one extra stop of 'film' speed.
I'd characterize the SD14 as my 'Kodachrome/Velvia' camera... sort of like a medium format manual focus/exposure camera. When I can take the time to think about a picture, the SD14 allows me to obtain superlative results in terms of color and resolution. The K20D is more of a 'chromogenic' camera in that I can get good results while having more latitude with exposure. Pixel for pixel (shooting a 100mm lens on the SD14 and a 50mm lens on the K20D, and cropping the same # of pixels out of the center of the K20D image), the SD14 is sharper.
My plan for the K20D is for 'action' type photography, like air shows, sports, etc. I'll continue to use my SD14 for landscapes, scenics, etc., or anywhere where I can be more deliberative. When I take the time to make a 2x2 multi-row panorama, the resulting 14 MP SD14 image blows any other dSLR away, not excepting the high-end Canons.
BTW, I think the SD14/K20D combo is worth considering. Both cameras can share the same batteries, use the same (M42) lenses, etc. And, I also think that Sigma and Pentax have both done a good job of putting forth the qualities (and implicitly, for Sigma, the limitations) of their respective products.
An example of the same subject with two different cameras:
Which one do you like better?