Unreal BSP editing tools are currently, hands-down, the most powerful bsp tools available in a traditional CSG-style level editor.
Unfortunately, the interface for dealing with brushwork remains fairly primitive and hasn't gotten as much love as Epic shifts to more of BSP foundations covered with static mesh detail. Radiant and Hammer remain far more speedy for building and texturing brushwork (and Radiant still has a pretty massive lead on Hammer there), but if someone were to add that level of construction efficiency to the Unreal tools... game over, man, game over.
Interested to see what direction id takes the editor for id tech 5, though.