I've had a fair amount of experience with Foveon sensor cameras. The first thing I can say is that sharpness is not equal to double a Bayer sensor camera. That is, unlike what Michael expects, this camera will not have the equivalent of 30 MP. It will be closer to 24 MP. Now, that's pretty good. But Sony and Nikon already have cameras out with that resolution. Later this year, Canon will up that, and it's possible that Sony and Nikon will as well.
But I've found a big problem with Foveon sensors. That is, they have poor color purity. Unlike Beyer sensors that rely on filters, the Foveon relies on the selective absorbson of light that silicon exhibits as the light passes through the chip. That is, as the light passes further through, different colors are absorbed, allowing each color sensor to reside behind the one in front. This is what makes the Foveon possible. But, that absorbson isn't perfect, hence, colors poison the other layer sensors, resulting in color mixing that simply can't be fixed.
In addition, the sharpness is higher in some colors, but lower in others, when comparing to Bayer sensors. While Foveon sensors have a higher resolution in the red and blue, it's actually lower in the green, again, when compared to Bayer sensors. That's entirely because Bayer sensors have two green sensing sites to one of each red and green, while Foveon sensors have equal sites for all colors. We are most sensitive to green, specifically a green-yellow. So, in practice, the sharpness advantage of Foveon sensors is highly dependent on the subject matter. Sometimes it appears a lot sharper, and sometimes less so.
When I first read about this camera, I thought that Sigma was making a pro model, in the way we think of pro models, and so even though the price was crazily high, I though it MIGHT be worth it for some. But upon finding that it's just a mid range advanced amateur body, I'm rather shocked. While their high end lenses are good, the rest are just what they seem to be. Unless they up their QC by a good bit, this entire senario is doomed, even if they lower the price.