I shot with a Sigma SD10 for the three years between my old M6TTL and my current M7. The image quality, especially the sharpness of the Foveon sensor was outstanding. The fact that the full quality RAW files were only 3.6M also really helped to stretch the value of memory cards.
The Foveon software was great, especially for correcting underexposed shadows and made for a smooth workflow.
There were also a couple of downsides. The most significant being that the Foveon sensor was TERRIBLE in terms of noise at any ISO rating above 100 (which is also the slowest). The noise pattern was really obtrusive. From what I understand this a side effect of the Foveon sensor's vertically stacked RGB strata. I never had much luck when using the popular noise reduction tools because of the noise pattern. I just left the camera permanently partked at ISO100. The new generation of Foveon sensors (in the SD14 and the DP1) is supposed to do better at this, though.
Something else to consider is that Sigma unveiled the SD14 in its supposed final form at Photokina last year but it only started shipping now. The DP1 was announced as a work in progress at Photokina but only shown in its supposed final form now. If the lead time on SD14 delivery to retail is any indication of the DP1's delivery, we may not see this until the start of the holiday buying season.
So... you guys that want one will have plenty of time to think it over!
I for one don't get the f4 lens speed. I think that f2.8 would have been much more in line with the general expectation of a high end ps, either film or digi.
John