Had XT then XTi, and took the big leap to 5D. I don't know that I will ever go back. I really like have a Full Frame and the color is amazing. I was concerned before I got the 5D about the size as I have some wrist problems, but was amazed at how quickly I adapted to the extra weight. The button lay out on the non Rebel versions seems more logical to me know as well.
On another note, my husband wanted to dabble with photograpy (he mainly does video) and had both T1i and 20D. He decided to go with the 20D because of build quality. He just felt that even if it was an older model it just had a sturdier feel to it. Although that does mean I think I have lost my 50mm 1.8.