For better or worse, here's my .02: The SD-14 has the best pixel-per-pixel results in the industry today. Unfortunately it has too few. On the surface this might seem like contradictory statements, but let me explain.
As far as picking up details and rendering an accurate representation of the light at each pixel location, the SD-14's Foveon X3 sensor is the best. At it's native resolution of 2640 x 1760, I don't think there's a camera out there which can beat it for lifelike quality. However, that's a small number of pixels overall. Yes, there truly are 3 times that number when you count all of the red, green, and blue pixels in the 'stack' in the sensor. However... and this is important, the native image is only 2640 pixels across. If you print out 8"x10" photos, this will work at roughly 225 dots per inch for your printing. If you've got to "go big" the only solution is to use Sigma's Photo Pro software, and convert the X3F RAW file to its 'Super Hi' JPEG image output, which comes in at a whopping 4608x3072. For all of you pixel peepers out there, that's more pixels than Nikon's D700 and D3 full frame CMOS sensors can generate! It's extrapolated pixels though, which means the image grows softer when viewed full size on a screen. Will it print better? I'm going to have to talk to the wizards at Imagemark in Phoenix, Arizona to find out for certain.
This particular SD-14 body I received appears to have a problem with focusing slightly too far on distant objects, when used with the Sigma 180mm f3.5 APO EX Macro DG IF HSM lens I just bought from KEH.com. Nice lens though! When focused properly on something, the details it can resolve are amazing. When the loaned lenses arrive from Sigma USA early next week I'll have a better idea if it's the new SD-14 or the 180mm lens which has the problem. It could be that the lens is slightly out of adjustment too as far as where the HSM is telling the camera it's stopped when focusing precisely. Whatever the cause, it's more than a little frustrating.
I hate going out on a limb and saying that the output from the SD-14 doesn't quite match the Nikon D300. Pixel-per-pixel, at 2640x1760, the SD-14 rules! At 'full size', the D300 rules. Since I was mainly shooting photos of a car at a normal distance, the advantage went to the D300, which appeared to nail the focus, if having slightly less finite detail in the overall image.
Will the SD-14 be better with different lenses? I don't know at this point, and the suspense is killing me!