For "The Resistance" how much do the Plot Cards add?
The plot cards add an entire additional level of logical thinking to the game, effectively making it harder for spies to perform. If you enjoy the logic puzzle aspect of the game more than the social engineering aspect, you'll probably enjoy the plot cards.
If you enjoy the "acting" portion of the game more, you might not like them very much. It's nearly impossible to make up an effective lie in a half second when you're trying to explain how it is that you're not a spy when person A to your left looked at your card and said you were a spy and person B to his left looked at person A's card and said he was not a spy,
and person A+B+C succeeded on Mission 2 but person D+E failed mission 1 in a game with 2 spies.
The cards are good, but if they're used carefully and come up randomly lucky, they can just end the game.