Worth the money, and only in alpha. Hands down one of the best PC games I've ever played. GREAT online community.
In this mode, the player is left to fend for themselves in a fun but often unforgiving world. Using ONLY materials mined by your own hands, you must build a safehouse to protect you from the horrors that await in the evening, when the sun goes down. With the latest update, you and a friend can do this together through a dedicated server which is incredibly easy to setup even for the technically challenged.
The graphics are a turn off for many, but in fact are the core component of the gameplay and will turn even the most hardcore "I MUST HAVE GOOD GRAPHICS OR THE GAME SUCKS" gamer into a fully immersed, terrified little child when the night comes and the zombies, spiders, and explosive creepers come after them. Go to the minecraft wiki page to learn more about the crafting recipies and gameplay, or just play the game and try to figure it all out on your own.
The game is 100% open ended. Other than your survival, your goal and gameplay style is limited only by your imagination.