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Read if image quality is your first priority
I'm a Nikon user. Have been since 1992. I just bought the SD14 with lens package here on Amazon. I only shoot RAW, whether it be Nikon or in this case Sigma.

If final image quality is ultimately your biggest priority, then the SD14 is a winner. The Foveon sensor is just incredible. I expect it to get even better in the future as Sigma has bought the Foveon corporation.

I received my SD14, and the next day, at 7:30am, went out to a local marina and shot 150 or so photos. Spent the rest of the day in Sigma Photo Pro software tweaking each keeper. The final results are simply stunning.

I own 2 Nikon bodies and 15 or so Nikon lenses. I'm going to hold on to them for awhile, but ultimately, I think I'll end up being a Sigma guy.

Ok, so the SD14 will have some downsides for most people. But not me. My Nikon D300 must have hundreds of settings. The SD14 has only a few. It is a pretty simple camera. So, if you like hundreds of options(and more control), then stick with Nikon or Canon or whatever. Also, mainstream software choices are limited to Sigma Photo Pro and Adobe Bridge or Lightroom.

But, man, if your goal is to shoot some photos and get great results with RAW, then try the SD14.

Pros:
Stunning photo quality after tweaking. The colors just pop.
Easy to use and powerful Sigma supplied software.
Simple in-camera menu
Few options to mess with(could be a negative for more advanced photographers)

Negatives:
Slow autofocus
Small buffer
Clunky body
Not really 14MP. More like 9MP.

A totally different breed of DSLR
This is a truly amazing and different breed of DSRL on market. It should not be your first DSLR to buy because it is not an all around camera to use (IMO). If you are planning to buy your first DSLR, Nikon D60 or Canon XSi should be a better choice since that's what they're designed for.
The Sigma SD14 has a very unique image sensor (Foveon X3) that can produce a very sharp and beautiful color image under sufficient lighting. However under dim light, images could be disappointed due to high noise and grainy. Using Sigma's own Photo Processing software to process the RAW (X3F) files is truly joyful.
The SD14 is a big improvement over the previous models (SD9 and 10) on design. The camera body feel very comfortable in the hand, shutter noise is nice and quiet, much improved battery system, a build-in flash... Besides, when you take off Sigma's unique dust protection filter on the body and add an infrared filter on the front of the lens, you can turn the SD14 into an IR ready camera and open up a whole new photography field.
Sigma is planning to have a new SD15 coming out soon so the SD14 has drop to an unbelievable price. It is really a joyful camera to use but remember that it has it's own drawbacks. Also you might find the choice of lenses are kind of limited. That's why I don't recommend it as your first DSLR.

Best Image Quality for the Price
I own a Canon 5D and Nikon D3 and my Sigma SD14's image quality is on par with both of these cameras.

However,...

... it does have a few shortfalls. Such as:

1. In low light conditions,... shadows will have "blotchy" noise and speckles of magenta compared to the same shot taken with a 5D/D3 at the same ISO settings. This can even happen in daylight conditions where there is a very wide dynamic range. The SD14 will expose correctly for the image, but the deep shadows will at times fail to render details in the shadows and those blotchy/magenta characteristics will appear.
2. Poorer AWB in mixed lighting than the 5D/D3.
3. Small buffer. If you need to keep up with action (sports, etc..), then you will be disappointed that after a burst of 6-8 shots the camera will lock up and take 5-20 seconds to write all that data to the card. And faster CF cards won't help. It is the buffer. The fact is that the processing is complicated and does not process quickly for fast shooting environments. Yes it will shoot 3fps, but the buffer is so small that you only get a little over a handful of shots and then the camera needs time to process that burst.

The positives?

1. ISO 50-200 in daylight conditions is spectacular. I prefer the SD14 over my 5D/D3 up to 200 ISO.
2. Image sharpness is amazingly crisp and has a 3D type feel that you can only appreciate if you have shot with this camera and a Canon/Nikon in the same shooting conditions. I have done so from the same tripod to compare shots taken with similar lenses in the same lighting conditions and can attest to the differences.
3. You can use Canon speedlites to use within wireless ETTL setups. I have two Sigma EF 500 DG flash units and three Canon 580EX II speedlights. I set the Canon's to ETTL slaves and the two sigma's to wireless and then the camera to wireless. You can set all of the flash units to either channel 1, 2 or 3, then match that with the camera's channel output to fire off all the flashes. The camera meters properly using this mix of flash units. I found all this out by accident. I was testing out the 5D and wireless flash setup VS. the SD14 and wireless flash setup. I had left the 2 Canon speedlights on while I fired off the SD14's and expected only the sigma EF 500 DG units to fire. The canons did as well and the SD14 metered for all four properly.

I'd have to go further into details to review all the pros and cons. I'll just say that as a 5D and D3 owner, the SD14 can hang in image quality. And in good daylight environments I prefer it to the IQ of either the 5D or D3. The SD14 is at a minimum, on par with both respectively and often trumps those two when lighting is strong.

Also, the dynamic range of the SD14 seems to be wider than reported. I don't know why. In lightroom I'm amazed at the amount of details I can recover in the highlights VS. 5D/D3. Maybe it is the fact that because the SD14 captures R,G,B on each pixel, those hightlights are of better quality and it is able to pull those details out of the highlights for that reason. I don't know. It just "is what it is".

Well worth the purchase. The far lower price point makes up for it's low light performance. Using flash or strong daylight environments, the SD14 can hang with ANY 00-00 DSLR.

SD 14- Wouldn't trade it for...
I sold my Canon 40D and lenses (Including L glass) to fund my purchase of the Sigma SD14. I miss the low light ability and the all around speed of the 40D. I miss the superior battery life and the ability to shoot full frame (If I bought another camera body) with the same lenses that I invested in for the Canon. I miss the stable operation (No lock-ups) of the 40D. The Canon 40D is an excellent camera, and is hands down a highly recommended option for many people who are into photography, I really do miss it in many ways.

But wait, this isn't a review of the Canon 40D, it is a review of the Sigma SD14. I would not trade back my SD14 with all of it's quirks (You must have read about them all if you got this far, and are interested in buying one) for the 40D. The reason is that when I look at my photos, I realize that nothing in my budget could get me anywhere near the quality of the images that I get from the SD14. I'm not a very good photographer, I'm still learning every day. But the learning experience is full of excitement with what this camera can produce. I don't care about all of what this camera cannot do when I sit down and have a look at what it can do- which is reward the effort of taking pictures with images that invoke everything from thoughts of "wow, did I really take that picture?" to a deep feeling in my gut that I really can one day arrive at my goal of producing images that reflect how I see the world.

For those of you who can afford to add the SD14 to your current system- you are in a fortunate position to explore this little gem without giving up on the stronger points (High ISO, high speed, stability, etc.) of your current camera.

For those of you who are thinking about getting started with digital SLR's, keep reading everything you can on the internet before making your decision, you will be giving up a lot of capabilities of the other camera systems, but if you have the same experience that I have had with the images, you may just find the SD14 to be the camera for you.

For those of you (like me) who would have to sell their current camera and lenses to purchase the SD14, all I can say is- I'm not in a position to tell you if it is the right decision for you, but I don't regret it for a second.

This is a DIFFERENT but Spectacular DSLR
Don't let the price or the soothsayers and complainers even suggest that this isn't a great camera......it is simply different, and different in a better way! I came to this camera from a Nikon D2X primarily due to the high prices of Nikkor Lenses......and trust me, the Sigma SD14 for the price is the best value on the market today. Just don't plan on shooting JPEG's......even the Sigma literature will tell you that this camera and it's processor were designed for shooting in RAW mode. I am totally impressed with the results, the color, the clarity and the low light capabilities. The supplied software easily converts from RAW to JPEG and is easy to use once you get the hang of it......then you can do additional post-processsing with Adobe or other image software, The possibilities seem endless and the more I use it, the more I love it!