Unity 5 update 
This low poly building pack includes the "NYC buildings pack", the "skyscrapers", "residential buildings pack", "warehouses", "Barcelona low poly", "Venice Low Poly Mobile" and many other models updated for Unity 5. 

165 unique building models in total, closed meshes.  
City demo Unity 4 web player (this scene is different in the U5 version) 
City demo Unity 5 for Windows download 
City demo Unity 5 for Mac download City demo Unity 5 for Linux download 

v. 3.0 -> Updated all materials for PBR and added emission maps for all buildings. 
Warning: rooftops are a lot less detailed than external walls.