I did this (10D -> 5D). I loved the 10D, but the 5D is a much better camera.
- The 5D is much faster than the 10D. Instant power on, writing to CF card, reviewing images, it's all much faster and smoother on the 5D. I was out making photos with my brother last week who still uses the 10D, and that remembered me on how long you have to wait for the camera to finish sometimes (the 10D).
- The image quality is ofcourse a lot better, but that's not strange because the 5D has twice as many pixels as the 10D. Images out of the 5D are a lot larger and sharper than of the 10D.
- Focussing on the 5D is more accurate and better in low light.
- The high ISO performance of the 5D is MUCH better. The 10D wasn't bad, but I always avoided ISO 800 and above on the 10D because of the noise. On the 5D I don't hesitate to use ISO 800 and 1600. The 5D has probably the lowest noise of any digital camera that you can buy today.
- The viewfinder of the 5D is really large and bright, very nice! I was struggeling with manual focussing on the 10D sometimes - on the 5D it is much easier to manual focus precisely.
- There is a whole list of other interesting features on the 5D: spot meter, ISO in 1/3 stops, Picture Styles, a better folder management system (upto 9,999 photos in a folder, instead of a new folder every 100 photos), RAW + JPEG mode, etc.
The 5D is a super camera. I absolutely don't want to go back to an 1.6x crop factor camera. I have absolutely no regrets that I got the 5D instead of the 30D, even though it costs almost twice as much.