I've read it, several times, in fact.
Excellent read, but a little unlikely.
Ever read the Book of Revelation? I don't think the scenario is too unlikely at all. In fact, I think it's a probability. The unlikely part has to do with survivors.
Still, speaking of survival, many of the preparations and disconnected events in the book, Luicifer's Hammer are very plausible. It was one of the main motivators for me getting interested in becoming a U.S. Army Infantry Officer and in getting myself ready to survive should things get that bad. I used many of my skills after Hurricane Andrew.
If you hop on over to the thread known as Threat Matrix, you should see a list of survival items drawn up by a freeper named appalachian dweller. This is an extremely comprehensive list. There is also on that thread a list of useful tips by me.