I would give this camera a 4.5 overall.
Buy this camera if you want FANTASTIC images and you aren't shooting a lot of critical-focus, high speed action shots. This camera can do that (action photography) but the AF system is not as sophisticated as offerings from Nikon or Canon.
One "drawback" often listed is that this camera ONLY works in RAW mode. Every image must be converted into a standard format (TIFF or JPEG) via the supplied post-processing software (Sigma Photo Pro), but the benefit you get is that extra level of control over the image. The software lets you automate the production of JPEGs or TIFFS, or you can go in and manually tweak all sorts of values that can turn ordinary shots into beautiful ones. In my opinion, this "drawback" is actually a feature because you will always have the raw image data to work with and as the raw processing software improves you can go back and revisit your photos.
The SDx series of cameras from Sigma are the only dSLRs that use the unique Foveon image sensor. This sensor is different from a standard Bayer sensor as used in every other dSLR in that each pixel location (photosite) has 3 separate sensors (1 each for red, green, and blue), whereas Bayer sensor cameras have a matrix that assigns 50% of the pixel values to green, 25% to red, and 25% to blue. Bayer sensors effectively provide 1/2 the given sensor resolution, i.e., a 6 MP Bayer sensor really has, at best, 3 MP of effective resolution, and the "fill-in" data provided by red and blue photosites is used to generate guesses and then the resulting data is up-rezzed to the given sensor resolution. Because each photosite only captures 1/3rd of the required data, Bayer sensor-equipped cameras must 'guess' what color is at each location, and while firmware and available computer-based raw image processing algorithms are pretty good there is an inevitable and unavoidable loss of resolution on finely-detailed subject matter.
There is considerable debate as to what the true pixel count is on this camera. Sigma proudly labels this camera as 10.2 MP (3.4 MP x 3), meaning there are 3.4 million photosites (discrete light sensing locations) but each photosite captures 100% of the available color information so there is no guessing. The ability of this camera/sensor to capture single-pixel detail is unmatched by conventional Bayer sensor-equipped competitors with more than twice the base resolution. Extensive testing shows that the SD10 image quality and resolution exceeds conventional 6 MP dSLRs and is very close to 8 MP competitors, but without the disturbing moire artifacts that are an unavoidable aspect of Bayer sensors.
What does all this mean? I can tell you that I have up-rezzed SD10 images (using third-party software with sophisticated algorithms, not the provided SPP with bicubic up-rezzing only) and the resultant images are as sharp or sharper than 'native' 8 MP images from competitive dSLRs.
If your primary usage was for photojournalism where you needed to produce lots of JPEGs that will never go larger than 8"x10", don't get the SD10... look at Nikon or Canon cameras that produce JPEGs in-camera.
What this camera excels at is fantastic, detailed photographs. If you are into landscapes, portraiture, or other areas where image quality is extremely important, if you are into the craft of photograpy, if you are willing to post-process raw images for the ultimate in photograph quality, then the SD10 deserves a serious look.