I have owned the Canon 1Ds Mark I for 4 years; it was an truly wonderful camera in its own right, but I was looking forward to upgrading to the Mark III and bought the first one sold on eBay on Nov. 29, 2007.
It was worth the money...this is a fabulous camera.
The 21.1 megapixel sensor is razor sharp; the new chipset is blazing fast, and the dual memory card feature allows you to put your raw images on one card and your jpegs on the other, simplifying iPhoto RAW/Jpeg workflow. This camera has all the software advances such as auto-rotate, replay zoom up to 100% actual pixels, and color histogram.
Liveview allows you to use the display as a view finder (albeit with manual focus), which can be handy if you want to hold the camera over your head to take a picture.
It's too early for me to tell whether the automatic sensor cleaning will solve the dust issues with earlier bodies, but so far dust hasn't been an issue in the first two months of use.
The battery life is amazing...much better than the 1Ds even though the batteries are smaller and lighter.
I like the ability to customize the filenames; you can specify the first 4 characters from the menu. By including the date or date/month in the filename, you can assure that your filenames will be unique even after you take 10,000 photos.
One thing to keep in mind is that with such a high resolution sensor, your lenses' limitations will be revealed more readily. While the 24-70mm f2.8L lens does work well with this camera, I've found that some longer zoom telephotos like the 100-400mm f5.6L and the 70-300mm f5.6 DO lens are too soft.
You may want to budget for a couple of new prime lenses to make the most of this camera's resolution.
If you own the 1Ds Mark II, then you have some of these features, and 75% of the pixel count, so this is merely an evolutionary change...for everyone else it's a revolution.
Note that as of January 15th, 2008 the raw files from this camera are not supported by Aperture or iPhoto, but are supported by Adobe Photoshop CS 3 and Lightroom.
Previous Canon cameras owned: EOS 1Ds mk I, 5D, 30D, 20D, D30
Canon lenses that I do recommend for this camera:
EF 14mm f2.8L II USM
EF 16-35mm f2.8L II USM
EF 24-70mm f2.8L USM
EF 50mm f1.2L US
EF 70-200mm f2.8L
EF 180mm f3.5L macro USM
EF 300mm f4.0L IS
EF 600mm f4.0L ISM
Lenses that may be too
Orignal From: Canon EOS 1Ds Mark III 21.1 Megapixel