I have had a 1Ds3 since December and have nothing but praise for it. I used to use a 5D which is now collecting dust.
The images straight off the camera do appear a little soft and as Canon recommend a bit more sharpening than previous is required. Do that and images just pop. Exposure, AWB and focussing are all excellent - the additional focus points really help. Dynamic range feels slightly greater than the 5D, although this is purely subjective - images appear to take a little more tweaking without degradation, for instance when pulling detail out of shadows (I am personally not keen on HDR).
Dust has been much less of a problem than with previous cameras - and I do change lenses often. I have found just one spot since December that would not just shake off. A quick clean and we're back to normal - and I have forgotten just how often I had to clean the 5D sensor.
I bought the camera as I wanted the high resolution for large landscape prints. It absolutely makes a better image that is easier to print at larger sizes. It does mean the best lenses are needed - I use Canon L series and a couple of Canon non-L primes without issue. It could well be nothing to do with the camera, but I've certainly more keepers now using the 1Ds3 than any previous camera.
2. Weight. It's big and heavy but carries better than a smaller camera.
3. Image size. Fantastic resolution comes with a price - disk space is filled much more quickly.
4. High ISO. Is not as noise free compared with some other cameras. The Canon 5D, 1D Mk3 and Nikon D3 are all better at high ISO, but then this is comparing apples and oranges. The 1Ds3 has much smaller receptors than any of the others it is often compared against so simply will display higher noise. That's physics! At ISO 200 or less I am delighted with the images. I'm sure comparisons will be quick to appear when the new Sony sensor is available.
5. Different card slots. It has one CF slot and one SD slot. There are plenty of options for saving to these - to both simultaneously, one after the other, RAW to one and JPEG to the other, etc. Why two different formats? What's wrong with CF only?
6. MLU is still nested in menus - BUT the custom menu allows it to be much closer than before.
As stated, I bought the camera for landscapes which will be printed large. I do not need high ISO capability, extreme shutter rates, etc. I want very high quality, high resolution images from a camera that can be carried without requiring a sherpa or donkey. I've got just that and I think its marvellous!