Looking through my 5D shots i would say that many, if not most, of them i could have taken with my 350D. Some though, i couldn't. The majority of these tend to be shots with shallow depth of field, as the 350D has neither the same DOF capability nor the big bright viewfinder (and EE-S screen) needed to properly position focus manually. In some cases colours and details just pop out in a way that the Rebel can't achieve, but i don't have so many of those.
So on to the pictures - both made possible by MF with the big bright viewfinder, the second incorporating the more shallow DOF of FF:
This is my almost two-year-old son who never smiles when I take his photo. This is as close as I get. This camera is best used when preserving family memories, in my opinion:
