Coming from a 50D you are going to see a difference. The 1Ds is a complete tank - slow, lacking an intuitive menu system, and the likes of picture styles weren't even on the drawing board back then. My 1Ds is very "green" - if I underexpose slightly or the WB comes out not quite right, images have a green tint - on the 450D I owned previously, images seemed to have a more red hue.
The main reasons I bought a 1Ds, despite it being a step back in many ways, are:
a) Full frame. This gives the ability to utilise the full DOF and FOV that any EF lens gives. No crop factor here - 50mm is 50mm - just like on film.
b) Sealing. I do some urbex in some pretty dodgy conditions and it just didn't feel right carrying a plastic fantastic 450D. The 1Ds would probably break whatever it hit rather than the other way around. Coupled with a 17-40L or another cheap weathersealed lens, and it's an excellent walkabout package for less than ideal conditions.
c) AF. One of the main things that bugged me about the 450 was its lousy AF. The conditions had to be right for it to work properly, and there weren't enough AF points for my liking. The 1Ds just focusses, and does it properly, providing the lens on the end isn't a 50mm f/1.4 USM (lol)
The 1Ds is becoming very long in the tooth now, but at the end of the day it's still viable in some ways. The 11MP sensor is still too much for me, and that's from a 7 year old camera. High ISO is simply non existant, with a measly maximum of 1250, so Canon's slower lenses that are easily usable on the likes of a 5D2 become a bit more challenging here.
One thing I do find about the 1Ds is the colour reproduction is decent. Without PP the pictures do look perfectly acceptable - on the 450D I found that in order to avoid the photos looking washed out, I'd have to use the "Standard" picture style, which actually ups the contrast and colour saturation compared to "neutral", which is what the 1Ds is based on. The images need very little PP to look good, in fact I shoot JPEG and only use a dash of sharpening (the antialiasing screen is very weak) and the pics are fine. You'll be surprised at how much sharper your EF lenses look on a 1Ds - a large, very low pixel density sensor will make a big difference.