Thomas Paine's fierce pamphlet is certainly a powerful piece of rhetoric; it is easy to see how his works rallied the colonists to the cause of war.
Paine is on solid ground when he attacks the legitimacy of monarchy as an institution; he skillfully cuts apart monarchist theory, showing how morally bankrupt it is for someone to reign merely because their ancestors won a battle.
Where Paine errs is in his advocacy of violence as means to end British rule. The base of his argument is that since British troops had fired on colonial militias, all legitimacy of the British crown had ended. Furthermore, because human lives had been lost, settling for anything else than full, total, immediate independence would be a "waste" of these casualties.
Here, however, Paine seems to blissfully not care exactly who fires the first shot. If violence by the other side is all that one needs to justify violence -- there will be no end to war! Paine goes on further to reject out of hand any concept of a negotiated settlement with the British, arguing that the British government is a snake that could not be trusted, utterly evil and corrupt.
I suspect, however, that Tory colonists, many of whom later found their homes burned, their possessions stolen, and they themselves brutally tarred and feathered and run out of the country, would apply the same harsh description to the rebels. Paine, however, cares little for Tories, whom he dismisses as traitors, even going so far as to call for their execution. He even encloses a harsh appendix aimed at pacifist Quakers who had advocated an end to the violence.
Furthermore, had Paine's own logic been applied at other times, the blacks of the U.S. South would have been fully justified in taking up arms when policement fired on civil rights marchers. Student protesters against Vietnam could have risen in revolt after the Penn State deaths. One reason neither did so, of course, is because they did not enjoy one advantage Paine boasts of - the advantage of home turf and the ability to outlast the British in a long-term conflict. We should fight, Paine argued, because Britain cannot win a war across an ocean. Accustomed as we are to thinking of Paine as a hero of liberty, is this really all that different from Mao's "freedom flows from the barrel of a gun"?