In the beginning the doors to get to UFO were pain in the butt. My soldier would open the door and than he will meet various enemies all in overwatch position. Moving out or shooting and the soldier was dead.
Than I discovered a cheesy tactic - one soldier opens the door, and the second one shoots. Than the first soldier closes the door, the second one moves out, and the third moves into his place. Than the first soldier opens the door, the third one shoots, the door are closed and third one moves out to make a place for the fourth one. This opening and closing of the doors makes storming the enemy base/ship a piece of cake.
Suggestion - either to make the door as grenades - that after activating they will open/close only at the end of the turn; or to make the door automatically open when the soldier is in front of them ...