In Letter 246, Tolkien writes that of all the beings of Middle-earth, only Gandalf might be able to defeat Sauron one on one using the Ring (Gandalf using the Ring, that is), thus vanquishing Sauron. Someone like Galadriel or Elrond might use the Ring and amass an army that could defeat Sauron's, but not forever vanquish him.
As has been mentioned, there is no real way to compare Boromir's taking the Ring to Isildur. Isildur took the Ring only as "weregild" for his father and brother, something quite important in the medieval real world, and thus Isildur does not simply desire the Ring for its power. Isildur certainly doesn't use the Ring's power, except when attacked by Orcs and he uses it to give him some chance of escape (which clearly doesn't work). Boromir, on the other hand, wanted to use the Ring to gain glory in war: that the Ring would leave him only as it did Isildur was beyond any hope for him and beyond his lack of nobility.
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"He was sent by a mere prudent plan of the angelic Valar or governors; but Authority had taken up this plan and enlarged it, at the moment of its failure."
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