In my opinion he most be the most over-rated and over-exaggerated figure in Irish history. During the so called War of Independence he led a small but effective counter-intelligence unit, assassins if you will. He adapted well to the urban guerillia war which was the only way possible to defeat superior military forces.
However he was wined and dined by the British elites, fell for their mind games and tricked by Lloyd George's rhetoric. While he clearly should have not been sent to negociate the treaty as he had neither the intellect nor the political experience, he still came back with the worst possible deal that could only lead to bitter civil war and partition. No treaty would have been better.
As soon as it became obvious that the majority of the people who fought against the tans and British army would not stand for it, the treaty should have been ripped up. Better to fight the imperialists than have brothers turn against eachother. Collins was utterly clueless in the time between the treaty being signed and the beggining of the civil war. He misjudged the revolutionary spirit of the IRA thinking they would accept such a blatant sell out because it was a "stepping stone". It's a pity he didn't have his brains blown out the minute he got back from England. The whole purpose of independence was so that the Irish people would have a better quality of life without British imperialism. The free state accepted British imperialism by continuing to pay British landlords rents for land we owned ourselves. The free state was no better economically for Irish people as it did nothing but replace British oppression with Irish oppression. Maybe they painted the post boxes green but they did nothing that would improve life for Irish people. In fact Ireland was worse off economically in 1930 than 1900.
And what did the treaty bring to Ireland? Nothing but poverty, the abandonment of our brothers in the north, civil war, executions, totalitarian government from people like Kevin O'Higgins, the continued dominance of the catholic church over our lives, oppression of women etc