Fun rocket-building & mission-planning game, it's suitably challenging and really imparts a sense of achievement. I cheered the first time I made it into space without blowing up...then when I made it into stable orbit...then when I orbited the moon...etc.
It feels rather realistic, though there are difficulty levels and different modes (e.g. Career where you have to worry about money, Science where you only worry about unlocking tech, or Sandbox where you do whatever you want).
Also, I never thought I'd laugh so much when my plans go HORRIBLY AWRY.