Unconditional must buy.

All I can say is WOW! What a great addictive game! kinda puzzley, yet has elements of cooperative and competitive play! This is probably one of my favorite 2 games!

UPDATE:
Now that I have several hundreds of hours, I can honestly say, this is one of the few games where you'll play the game again and again, and use the achievements as a viable way to add unique complexity! I also have been recruiting friends to get it to play with them! So fun watching people have the epiphany of discovery for the first time! Or maybe play with fellow veterans and make the world extra dangerous (deathworld / marathon) and build something beautiful and enthralling.