I always thought it was a pretty good metaphor and didn't understand why he got so much grief for it. Breaking up messages into little chunks, sticking each chunk into one of those little capsules like they have at the bank, putting each capsule into a pneumatic tube, sending it down a series of tubes with switching stations along the way, and reconstructing all the chunks (which may have taken different paths through the series of tubes and may have arrived out of order) at the destination -- this is a good metaphor for the internet!