In a word, Very! The H1N1 Swine flu virus is very contagious and spreads quickly from human to human. Though its rate of mortality and symptoms are much the same as common flu, its rate of contagion is its chief point of differentiation from common flu.
What also makes it more easily contagious is the fact that we have not yet developed immunity to the disease; the antibodies that routinely fight of disease in the human body are not yet developed as well to fight the H1N1virus. This is also the reason that immumo-compromised persons, or persons with lowered immunity are more at risk from the virus.
A person who contracts the swine flu virus can become contagious even before they themselves start to exhibit any signs or symptoms of the disease. This is to say that a person may become contagious even before they get sick. They remain contagious for 7 or more days after contracting the virus.
The way that Swine flu spreads is much the same as regular flu; it spreads through coughing, sneezing, or touching something that has the live virus on it.
According to this picture sourced from Osh.net, Swine Flu is also different in that it speads from person to person and is not limited the way other flus are.