Love building stuff and see it explode? This game might just be for you. You're into Aviation and/or Space? Even better. Love extensability? There's a ton of mods too. What is it about? Fiddle together a rocket and try to reach space. Fail (or not) - refine your vessel, do it again. Get better every time. Go to the moon - you'll never forget your first moon landing! Explore other planets. Or don't. Just build something and see if it flies. Want a challenge? Choose carriermode where you have to do contracts to get money to pay for parts and research. The sky ain't the limit - the solar system is (or not - have I told you about mods?) With BDArmory you can even build main battle tanks, dive bombers, biplane fighters or mechas and let them fight or fly in formation. There are people with >1000 hours played that haven't gone to space yet as you can have so much fun on the ground too. Even if you're the hardcore space nerd, with "Real Solar System", "Realism Overhaul" and NASA/Ruskosmos-part mods you can simulate missions on a realism level never seen in a space game so far. I suggest playing Vanilla until it does no longer challenge you. Once you want to start with mods, use CKAN - it handles mod dependencies and saves you a lot of trouble.
