[Marvolo]

Cell ended perfectly because the story was over honestly. The story concerned him getting his son back. He got his son back, end of story.

The Dark Tower series was about him getting to the tower, it consumed his life and became the obstacle of emotional growth and morals along the way. The tower sent him back to the beginning to go through it again, so its champion would make the tower full of love, not of death, because that I think is the key to saving Roland's world. Roland was supposed to be a Knight Errant, questing towards the Tower as a last bastion of the world of light. Instead he was a deathbringer, destroying entire towns and murdering along his way. His cold logic and quick hands dealt with every situation and only changed towards the very, very end. So he will do it again and again until he finally gets it right. I personally think the tower is Dark because of him. But that is just me.

The endings are good because they are honest. A lesser author would give people the sugar coated version because they are worried about book sales for the next book. He isn't worried about that, so we get a good, honest ending.