I consider the $10 DLC Leviathan and the $2 Firefight DLC, really a weapon pack, worth the money. In the weapon pack there is one Adas Anti-Synthetic rifle, if one wants to carry on playing the game at higher difficulty this rifle makes a lot of fights much easier, supply your mates with this rifle and let them fight
As expected they do not change the endings but provide hours of entertainment. DLC in general are less value for money, considering the hours of play, than the original games.