I've found that the level of detail and the sharpness of the images is outstanding once you follow proper technique and sharpening methods.
I quite agree. Four points:
1. Images do not appear as sharp with previous standard sharpening settings. A little more is needed and the images just pop. This is something Canon advise. Just be careful not to over-sharpen though.
2. With the increased resolution any lens that is not 100% will show up as softer images or with more noticeable issues than maybe with other cameras. I mostly use L series and have had no issues with 85 f1.2, 70-200 f2.8 IS, 24-105 f4 IS, 500mm f4 IS, but I did try an older 70-300 to see what it could do and suffice to say I will not be repeating the experiment on that camera. My 50 f1.4 and 100 f2.8 non-L lenses are also fine.
3. Camera shake is more obvious with the 1Ds3 and a stable hand, tripod or very fast shutter speed is essential. I suggest that the old "rule of thumb" about 1/focal length as minimum handheld shutter speed should be halved - so a 1/100sec becomes 1/200sec for a 100mm focal length. Image stabilisation does of course help.
4. Auto focus is different in the 1Ds3 than a 5D or 20D or... Take a little time to get used to how it works. I have not had any issues with auto-focus under "normal" conditions, but always use manual focus for macro work and for much of the time with the big lens.
Hope this helps - I have found the camera works beautifully and delivers wonderfully sharp results. I cannot recommend it enough if it provides the specs you need (but please lets not go into that debate here!)