I thought it was a really interesting episode. The other man, who I’ll call Esau, appears to have been plotting Jacob’s death for a long time.
When Locke went down the well at the site of the future Swan, after he was separated from his friends, he saw a ‘vision’ of Jack’s father, Christian Shephard. I believe this man was actually Esau. The Christian-apparition told Locke that Locke not only had to bring them all back, but that Locke would have to die to do it.
This fits perfectly with Esau’s plans. Since Locke is the current leader of the Others, he is the only one who is allowed an audience with Jacob. Esau knew that Jacob did not have the ability to resurrect the dead, but no one else knew this (although Richard Alpert, who seems to be the most in-the-know, is visibly surprised at Locke’s supposed resurrection.) So Esau used this opportunity to take on the guise of being Locke, and in doing so, guaranteed an audience with his reclusive rival, Jacob.
As to the flashbacks in which Jacob meets with Kate, Sawyer, Jack, Sayid, Hurley and Sun/jin, I’m not sure if I believe that was really Jacob in every instance, or if it was actually the devious Esau. In Sawyer’s flashback, we see Jacob facilitate Sawyer’s vengeful nature by offering him a pen to write the infamous letter. And in Sayid’s flashback, we see him appear to Sayid at the very moment that Charles Widmore’s assasins killed Nadya (which suggests he knew the precise details of how the assasination would take place), yet he did nothing to warn Sayid.