Looks pretty good, but the sizing and angle are off enough to give it a strange appearance. The relative size of the eye on the right is too small.
Obama was looking to one side and maybe a little up and mccain is looking straight ahead and maybe a little down. The nose splits the face about 25/75...and yet the tip is pointing straight out...not the slight profile it should have at that angle.
Though this can't really be fixed unless you get photos of them both with their heads at the same angle.
Overall, the PS job looks good...hard to see the "McCain layer" edges unless you stare at it for awhile.