Mission: SPACE is one of the most popular attractions in the park and you will want to visit this pavilion early in the day or get a FASTPASS to decrease your time in line. Even in the slowest of days, you can expect hour long lines for this amazing event.
Mission: SPACE is located on the left hand side of Future World as you enter the park. The main attraction is a simulated launch and trip to outer space. There are two versions of this attraction since there have been some injuries and complications in the past. The more aggressive version has the “astronaut” feeling several Gs during the flight. The less aggressive version doesn’t put the “astronaut” through this experience but still allows them to play a part in the trip with a very specific duty. You are well warned before entering the attraction and several people have been taken to the hospital due to health concerns that became aggravated in the aggressive ride.
After the trip, you can spend a large amount of time in the interactive game area that has team play for a large group of people. Emphasis is on working fast and 
teamwork. This area is one of the better multi-player games in the park and you should spend the time enjoying it.
These are just my opinions. Other people may have other opinions and here are some:
Mission Space at EPCOT
EPCOT MISSION SPACE VIDEOS