Words alone cannot express how amazing KSP is.
To be honest, this is not a game for everyone. If you are in the market for some fast-paced "space combat sim" with all kinds of weapons and big explosions, then this isn't what you are looking for. (Except for the big explisions... KSP has you covered there).
However, if you ever dreamed of actually being an astronaut, this is definitely for you. If you sometimes forget to breath when watching recordings of the Apollo moon landings, this is definitely for you. If precise, measured flying; careful planning; and delta-v budgeting are what drives you, this is definitely for you. If you want to learn about Orbital Mechanics and are dying to know what the Oberth Effect or a Hofman Transfer Orbit are, this is definitely for you. If you like Physics, this is definitely for you.
Optionally, if you just like building rediculously large rockets and watching them explode with glorious comedy when they fail catastrophically because you forgot ONE PART... then this is also definitely for you.
Kerbal Space Program is challenging, fun, frustrating, mind boggling, huge, hilarious, complicated, deep, and just plain awesome.