Great Sci-Fi story, beautiful graphics, unforgettable ending, good voice acting - a very solid adventure game overall.
And when you match a story like that with great graphics, gameplay mechanics, humor and good voice acting you're in for a threat - it's as simple as that.
There are a few puzzles that are mind numbingly hard/tedious towards the end, but be prepared to give it 30 min and then maybe use a guide and it'll be ok. Fans of anything Sci-Fi and adventure games owe it to themselves to buy this one, I can't imagine that you'd be disapointed.