As with the acquisition of any new camera equipment, I test it out on my ever unwilling model, who most often is roused from a contented nap to do me such modeling favors.
It was going to happen sooner or later - I caved in and got myself the Canon 5D Mark II. It's UPS delivered arrival is anticlimatic, as the battery is yet to be charged, straight out of the box. For my purposes, the main advantage of this camera body over my existing 40D is low light performance. Conveniently, my model lay snoozing in the north-facing living room, excellent low-light conditions in which to do a quick comparative test. Same 50mm f/1.4 lens on both cameras.
The first obvious difference is the Full Frame-ness of the 5D, as opposed to the 1.6 crop factor of the 40D. Crop factor (I think) results from a smaller sensor (what I imagine to be the panel that takes in the image). So when you slap a 50mm lens on a 40D, in fact is it more like using a 50mm x 1.6 = 80mm lens - you get that much more zoom. Thus with a 5D and other Full Framer bodies, you get a wider frame. The practical application for me is that I don't need to back up as much to fit in more dog/ baby/ family. Oftentimes there isn't enough space to back up into. But there is an advantage to having instantly more zoom - one can maintain distance from a snot spluttering child, for instance.
Second obvious difference was the clarity of the playback images on the back of the camera of the 5D. This matters because I can see right away and for certain if things are in focus. On the 40D, I'd reason, it seems focused enough, and hope for the best. Usually it turns out fine, but it is nice to have this reassurement.
On to image quality, and the 2 photos above are a poorly set up experiment, as she's refused to maintain the same lounging pose for the 2nd picture and only bothered to turn and glower at me over her shoulder. You can see the diff between Full Frame (1st pic) and 1.6 cropped frame (2nd pic). These are unaltered pics, aside from cropping into squares. In this low light, you can see that the resolution of the first is much better than the second. Focus on the eye in both cases, and in the 2nd, struggle with low light is pretty obvious.
Ah, and so begin my adventures with my 5D! Poor 40D, what will become of you. I think I may use 40D for beach shoots, which fill the camera with dust. And definitely anytime I need rapid firing of shutter; 5D is notoriously slow - ie. not for sport photography. And I've yet to even venture into the world of VIDEO with my 5DII.