Even more heart, more polish, new issues
The Only Traitor is a side story that in the end becomes one of the story inputs in the main game. The DLC's main character travels with his car, looking for a shelter from the Second Visitation.
First of all: gameplay-wise, it is better than the main game. You can see that the devs carefully listened to player feedback and removed or heavily changed the most problematic parts... while introducing new issues.
The management bit while travelling is now gone. You can just learn the story of your current companion and craft ammo and medkits and then decide it's time to arrive to the next destination. The enemies are more varied than in the base game. Fighting now requires more precision than patience, and you learn more of the backstory, and in more sizeable chunks (which helps).
And finally, the DLC's soundtrack is, like, totally awesome.
However, there are issues, too.
Even though the protagonist's baseball bat is a stronger weapon than Edward's fists in the main game, I was happier with fighting for Edward. I knew exactly how to use the fists, while with the bat, I never learned for sure how the swinging worked. Often it happened that an enemy that I'd normally one-shoot would surprise me with surviving the attack. Also, charging the strong melee attack failed even more frequently than in the base game (originally, I blamed my old mouse, until I found out that other players had the same problem).
Also, even though we learn more about the First Visitation and the Council, especially the role of Arthur Vane now makes even less sense than in the base game. Unless you simply accept that there's no point trying to understand alien logic. But then again, why bother learning a story that doesn't make human sense?
Technically, The Only Traitor improves on the main game in most areas, has a great soundtrack and explains more of the story. If you liked the base game, you should definitely get it. If not, it wouldn't change your mind.