I just bought one to go with my 5DMk3. (I had a 5DMk2, too, as backup which I just sold.) I wanted a low-ISO tank-like machine. Do some reading on the web: See the comparison files offered by
www.imaging-resource.com, because they have shot the same setup with many cameras, so I could look at a full-size file of a still life with both 5DMk3 and 1Ds-Mk3. The older camera clear winner at ISO200.
Bought a body for 2300 delivered way out here in the Pacific. Seemed fair. My experience in the last two weeks is good but for two things, one I knew about and the other a bummer. The noise picks up fast at and above ISO400 -- in the shadows. ETTR is really helpful with this sensor. The bummer I have written in a nearby thread about an error 30 message when using the 1Ds with LiveView. I think these cameras have a high rate of issues with LiveView. I admit I am reading between the lines here but the Canon tech guy asked me right away, "Were you using LiveView?" So my copy may be $300 more. That's taking the cost to the same as a low cost or refurb 5DMk3. Some people never have an issue with this camera. I did. "Sample of one" here. And the guy who sold it to me, I know, never used it with tethered LiveView.
Be aware that the focus is not lightning fast... as someone with a not-so-good copy of 5DMk3 I wanted good focus. But at least it always focuses.
On balnce, I may feel the experiment still came out OK. I may never see the error again if I avoid live view. But that's one task that I wanted the camera to do.