I've had a 1DS MKII for a couple of weeks now, and generally very impressed with it.
I've printed out some A3 size prints (canon i9100) that have blown me away regarding detail and definition.
My computer groans processing the RAW files, so I generally shoot RAW + small jpeg, which means I can scroll through the jpegs (on computer) and cherry pick the RAW files for processing, I also find the files are more tolerant of post processing (i.e. you can push them quite aggressively if needed).
It's a beast of a camera, therefore I use a Lowe pro toploader zoom when out walking (normally the Yorkshire Dales), I find this a lot more comfortable than it hanging round my neck. I use a 17-40L mainly, so can't stack filters, and a normal polarizer causes vignetting at full frame (I'll need to invest in a thin model)
Although most of my photography involves shooting at ISO 50/100 (tripod), I'm impressed with the high ISO results.
I found the menu system complex at first, but after a few days quite intuitive, irritated that you have to go to menu for mirror lock up though.
I've also found my post processing workstream has changed (previously a 10D), and I'm still experimenting to what works best.
All in all, am I pleased I bought it. yes definitely, is it worth more than 4 times the price of a 20D, probably.
Regards
Les
http://lesmclean.photoblink.com/
Canon 1DS MKIII,7D, 85 1.2L, 24 F1.4L, 135 F2L, 200mm F2.8L,50mm F1.4, 120-300 F2.8, 12-24mm f 4.5
http://www.lesmclean.co.uk/
Concentrate on equipment and you'll take technically good photographs. Concentrate on seeing the light's magic colours and your images will stir the soul. - Jack Dykinga