The projected orbit after perihelion should be more or less the same, whether it’s still one intact object or the center of mass of many pieces. How bright it’s going to appear as it moves further away from the Sun will depend on how much damage was done, i.e. how much of the ice “coating” remains, etc.
ISON was predicted to come within a few dozen million miles of Earth around December 28th, so I’m sure we’ll get a good picture of what happened to it from the various astronomy sources by then.
Will definitely get out the telescope but when I do, it rains.