Is Minecraft as great as everyone says it is? Yes. Yes it is. Still in early alpha stages, it's not even a complete game, but its core game mechanics are so addictive that, as of this writing, it has sold over 171,000 preorder/alpha access passes at a discounted rate.

The game desperately needs documentation and in-game tutorials, but until then, there are plenty of fan-made tutorials on YouTube. Here's a good one:

If you're not sure you want to get into it, you might still want to try Minecraft Classic, an earlier version of the game with no creatures and unlimited build ability that you can play in a browser (unsigned Java applet) for free. But the latest alpha is already worth the few bucks the author is charging, and those few bucks get you every future version of the game going forward.