I've now spend a couple of weeks shooting with my new 1Ds and thought it was about time that I gave some insight into the transition from my 5D to the 1Ds3. I've mostly shot with primes (28, 50, and 100mm) though the 24-70 zoom has also done very well. At least with the primes I don't feel like the lenses are limiting the sensor at all, especially when they're stopped down a bit.
Let's get some of the obvious stuff out of the way first. The 1Ds is bigger and heavier than the 5D, and by a decent margin. It's not heavy like a Mamiya RZ67, but it's not the camera I'd want to carry around all day with a heavy L zoom while sightseeing in Paris. The files it produces are obviously much bigger as well, so hard disk space gets eaten up twice as fast. I've started being a little more stingy with my RAW files, keeping only the ones I know I'll actually use, versus another 50% more that I used to think I'd get around to processing but rarely if ever did.
As for the RAW files, they're great. Resolution, with a good lens and technique, is stunning as I've noted a couple weeks ago with the boring and ugly leaf example. There is an improvement in the malleability of the RAW data, but honestly, not as much as I thought there would be, at least on the highlight end of the spectrum. There is however room to pull up shadows without much noise, especially at low ISO. As with most digital systems, under-exposing is generally a better idea as the clipped highlights kill ya. That said, the highlight to white transitions are much less jarring, with plenty of data to pull back with the highlight slider in Lightroom. I'm caulking this up to the 14bit files.
While there is more detail overall, I think the greatest use of the glut of information is to give the image noticably more texture, at least to to my eyes. They feel less 'digital' (that being the negative plastic look that people complain about) that the files from my 5D and I've also noticed the images need less post production overall, seeming to have more character right out of the camera.
Noise levels are very good, I'd say about the same as my 5D, which was pretty good as well. I'm not a sports shooter so I don't need 1/500th of a second at iso 12000, but I regularly shoot at 800 and 1600 and have no problems to speak of. Plus remember that with that many pixels, you can theoretically use extra noise reduction then down sample and sharpen and end up with a 12MP image that's really low noise. I have had no need to do this though, so it's just conjecture. ISO 100 shots do seem to have a little more randomness to patches of color (a blue sky for example). You could call this noise I guess, but I think it's a good thing because it's very subtle and makes the images feel much more like really good film scans. It's much like dither in a digital audio recording if you have any experience with that.
The shutter is fast and tight, mirror blackout minimal. None of that gets in your way. And the viewfinder is butter and probably my favorite part of the new camera and the hardest thing to go backward on. It's big enough to actually manually focus with. Even the 5D finder feels small in comparison, I can't imagine what the step back to a rebel would be like. After using the 1Ds for a couple weeks I wanted to test some tethered shooting scenarios using Capture One and so picked up the 5D body for the first time when I hit a snag. And wow, in comparison, the 5D feels really tiny and something akin to a toy. Not that it is or doesn't take great photographs, that's just the visceral first reaction. I also like the idea that there are no dummie modes or anything on the camera. That you're average person would have no idea what any of the buttons do (I'll admit to being a bit of an elitist in that way)
If they could fit the 1Ds sensor and viewfinder in a 5D size body, that would probably be my ideal camera. But that's not really an option, so I'll take the trade-off. As of right now, I'm much less likely to pick up my Hasselblad if I want to take high-end images, the files I get out of the new camera are more than a match for medium format scans to my eye. Much more so than the 5D.
If anyone has any specific questions, I'd be more than happy to answer.