I have read several blogs discussing whose side Charles Widmore is on. I think after watching the last episode, I might have some idea. The truth is, Widmore is on no one's side but his own. I think Widmore came to the island because he wants the Source. Jacob's mother mentioned that the source is in all men, but they always want more. It looked like as the man in black drifted into the cave that some sort of electromagnetic event occurred - it looked very similar to the other electromagnetic events we have seen on the island. I think this is why Widmore brought Desmond back - because Desmond is the only person who can go into the cave and not suffer the fate of the man in black. He is the only person who can reach the Source. Just my opinion, but I have a hard time finding Widmore to be an ally of either the candidates or the man in black.
By the way, now that we know that Jacob and the Man in Black are brothers, it throws the tile of the Gary Troupe novel Bad Twin into a whole new perspective doesn't it?