The strongest of these three missions has you disarming a bomb in a train station. Unlike the bomb disarming mission of the core game, there's no way to extend the countdown. When you get dropped down, the situation is quite precarious, with
Zhang's 'story' doesn't really amount to much
These are nice bonuses to get, and may adjust your approach to the overall campaign. However, beyond the three missions, there isn't anything that really adds to the game. Given how short these three missions are (about an hour and a half of content), a new weapon, enemy type, or ability would have gone a long way in making the add-on seem like a better value.
At $7, Slingshot is ultimately an expensive unambitious expansion to a terrific game. While I hope Firaxis'
You'll finally get the opportunity to fight aboard a flying UFO