I bought a 10D about four years ago, and still use it regularly. Earlier this year I purchased a Rebel XT mainly for it's small size and also because the price was right. Then I found a pretty good deal on a 5D. Now I have three bodies. I am going to sell one of them. I guess I am asking for opinions in addition to the ones I have outlined.
I really don't have a need for three bodies, I even wonder about keeping two at times, but I do like the idea of one crop and one full frame body.
I love the 5D because it puts my lens' field of view back to what I learned on. My 50mm is a 50mm again. I am impressed with this camera in every way.
The XT is simply OK. It takes good photos, and I like the size for certain circumstances. I like the newer technology of the XT body, and the faster start up. Although it's pretty easy to use, I find I miss the second dial at times, I always have to look to make sure I am pushing the right button to access the functions accessed by the second dial on my other bodies. It uses different batteries and a different remote shutter cable. I don't have a quick release plate for this body yet.
The 10D was my first DSLR, it still takes great photos. It uses basically the same ergonomics as the 5D(both bodies basically handle the same way). The battery is the same as the 5D, the remote shutter release is the same as my 5D. I already own a Arca plate for it.