I own and use a Sigma SD-10. I'm pleased with it well beyond what I ever thought I'd be.
Having said that, I'll also add that it's not for everyone. If you are looking for outstanding color images, easy-to-use intuitive processing software, and a camera that will challenge your creativity every day that you own it, it's great.
If you need essentially a point-and-shoot camera that produces almost instant JPEG images, it's not for you. Canon and Nikon make several dSLR's that fit that niche much better.
The SD-10 produces only RAW files which must then be processed using Sigma Photo Pro software (supplied free with the camera - generally acknowledged to be the best on the market for OEM). They can be batched in Auto so that it takes less than 10 minutes for, say 60-70 images (512MB), but if you are like most SD-10 owners, or one who's thinking about it, then you know that that won't happen either because you'll want to do "this and that" to the images to explore their full potential.
I shot 35mm for 40+ years. I spent just over 2 years exploring digital cameras. I then got rid of all of my film gear and went "cold turkey" with the SD-10. That was traumatic, to say the least. What I've received in return are images now that I can print in my home office/digital darkroom within a hour or two after capturing them (10 minutes in a pinch). Those images now get more "Ooohs!" and "Aaahs!" than I ever got with film. I was skeptical about the Foveon sensor and the SD-9. It took a lot of flack from other dSLR owners over on dpreview, and some of it rightly so. Then the SD-10 came out, and it was a whole new ball game. The Foveon sensor produces images which appear to have somewhat of a 3-D look to them. My customers always bring that up when they look at my portfolio. I accentuate that look of apparent depth by using the Epson R800 printer which has been reported to "layer" its ink droplets more than other printers on the market, giving even more of a 3-D look - one compliments the other. Together they produce great visual images.
I've been experimenting with this camera since I first got it last April. Actually I'm on my second camera body. The first one was defective - it happens with all camera manufacturers. Sigma worked me through some tests on the telephone, determined that it was the camera and replaced it quickly. I cannot say enough about how excellent their customer service was/is. The second camera body has operated flawlessly.
There is almost a limitless variety of combinations and permutations that one can do with the camera settings. It's awesome. Right now I've pretty much settled into a workflow that uses TIFF 16-bit files. I only use JPEG when I have to post an image on PBase. I'll post my PBase account here, but, PLEASE, note that none of the images posted there have been cropped, retouched or postprocessed and several were done with the defective camera body. They are strictly Auto batched with a slight tweak sometimes to Sharpening (+0.5) and Saturation (+0.2), or in the case of the Negative Fill Light gallery to Fill Light. As such they are "snapshots". They are all "dumbed down" JPEG files. When I print, I use TIFF 16-bit files. I do cropping, retouching, etc. before I save and print. There is a tremendous difference in the images. I really hate to post them, because what you see there is not anywhere close to what I print for my portfolio or for my customers, but they are here. I had to learn how to use PBase sometime...
www.pbase.com/cjmax
Currently, I'm waiting for my upgrade to Photoshop CS and my new Wacom Intuos 3 tablet to arrive. My current version of PS doesn't handle TIFF files adequately. I'm really limited as to which tools I can use, and a mouse is a painful way to do retouching. When PS CS is up and running, I plan on redoing my PBase site with more professional images. I grimmace when I write that because I know that those images will be "dumbed down" too, but that's the way it goes.
For those of you who wonder about my workflow (probably not many) it is: Download from camera to cpu and Sigma Photo Pro directly using FireWire connection, process RAW files in Sigma Photo Pro and save as TIFF 16-bit file, postprocessing (retouching) if any done in Photoshop and saved as TIFF 16-bit file, TIFF 16-bit file cropped and printed in Qimage.
I download directly from the camera for 2 reasons: 1. the FireWire connection is as fast as it gets, 2. I've heard too many expensive horror stories about people bending pins when taking CompactFlash cards in and out of their camera and card readers so I wanted to avoid that scene. I'm here to enjoy my camera, not to enrich repair departments.
I actually use Photoshop as little as possible. Why? Simply because I lack self-control. I've found that it's too easy to get drawn in by, "What if I do this, or that, or the other thing here?" Before I knew it, it was 3 hours later. I can't make money that way - not that I'm making much anyway. I also use Qimage for cropping and printing. It's just easier than using Photoshop. I set all of the settings to give me the best quality image once. Then I just choose my image size and Qimage does all of then needed work for me. I don't have to enlarge in 10% increments to get the best results as PS requires.
How does the Foveon sensor compare to the Bayer pattern sensors? This question is always a good way to start WWIII. There is no way to do a direct and fair test comparison simply because the Foveon sensor samples RGB colors at each sensor site while the Bayer pattern sensors use 4 sensor sites to sample black and white which it then processes into RGGB colors. That accounts for the sharper Foveon sensor produced images. The jury is still out on how many Bayer pattern pixels the Foveon sensor is equivalent to. At first the critics said that in the SD-9 it wasn't as good as the 6 megapixel Bayer pattern sensors, in the SD-10 it was equal, now the same critics are saying that its almost as good as the 8 megapixel Bayer pattern sensors...I don't think that they really care to say. Just take a look at who spends most of the advertising dollars with them, and you just know that they aren't going to bite the hand that feeds them. By the way, those additional 2 megapixels went to producing better green colors and a few other color issues. They don't have anything to do with resolution.
My suggestion is that you take a look at whatever cameras you are considering, but don't stop there. Look at some 8"x10" or larger photographs and compare the images. 4"x6" is too small - don't waste your time with them. I'd look at TIFF 16-bit file images too. They are better than JPEG q.12.
Well, I've rambled too much here - got a nasty cold, and I'm not thinking too clearly. If you have any questions, ask. If you're not too far from me, you're welcome to take a look at my SD-10 and some of its images.
Cliff.