I've read a couple of articles on it recently. Should be a neat sight. I don't think an impact of any sort is very likely at all.
Well, the nucleus of ISON is supposed to be around 5KM. Pretty large, but not exactly the same as an asteroid. It is likely to be very loosely organized, and mostly ice and dust. A comet that size would be spectacular (to say the least), but may not even reach the Earth's surface. Would be one HELL of a meteor shower!And if there IS a big impact... Who cares?
That's one of those instances where being a "fatalist" certainly pays off.
-J-