I upgraded from a 10D to this 5D. Wow. A HUGE difference. The 10D is a good camera, don't get me wrong, but the 5D really puts it in its place.
I started out using 35mm and I convinced myself over the years that a 1.6 crop factor isn't really a bad thing. Actually it's not, but it is nowhere near really good. I am very happy to be coming back to full frame as I always hated two things about APC sized sensors:
1. Trying to get even a moderately wide-angled shot inside. (I always found myself squeezing against a wall or in a corner just to get a decent shot.)
2. The fact that to get wide-angles you had to buy either a very expensive piece of regular glass (14mm) or a lens that only works on certain DSLRS. The more a product can be used on more things, the happier I am.
So I am happy with the full frame sensor.
I can't add much more to what others have already said, but I will mention one other HUGE difference I have found between the 5D and the 10D: correct focus.
Way too often I thought that I had acquired a bad 50/1.4 and a bad 100/2.0 because of consistent back focusing on my 10D - seriously noticable at wide-open aperatures. At first I thought it was the camera, then maybe it was me, then I suspected the glass, but after some serious testing involving a very sturdy tripod, a ST-E2, inamimate objects and my Elan 7E as the control, I knew it was my 10D.
I actually became quite good at knowing where on a person's face to focus (tip of nose) so that their eyes were in focus when using wide-open aperatures. But that was before the 5D.
I now know the pure joy of being able to get my glass to focus exactly where I want it to focus. My 50/1.4 focuses dead-on at 1.4. My 100/2.0 is much much better and I can see why now so many people like the 100/2.0. It's a pretty sharp lens.
Anywho, if you have the cash - or the ambition to save up for it - get the camera. You won't be disappointed.