Boromir tries to take the Ring by force from Frodo, something he would never have done in his normal mind. So he was definitely under it's influence at that moment. And just as definitely managed to break that influence alone and unaided - a truly extrodinary feat that would probably have been beyond the strength of most Men or other beings.
I cannot recall anybody ever explaining to Boromir exactly why the Ring cannot be used, as Gandalf explains to Frodo, it is simply stated as a fact at the Council and he is expected to accept it. And apparently does at first, it takes months of the Ring's influence working on him to move him to try to take it - and yet for all that he breaks the spell almost immediately.
As for the greater corruptibility of Men; Aragorn and Faramir both refuse the Ring, Boromir attempts to take it while under it's power but then breaks free of that power and repents his actions. Seems to me 'weak Men' do pretty well - far better Saruman the Maia and no worse than Galadriel and without the light show.
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