I bought this amazing game several months ago, and I am addicted. This game has everything. Infinite worlds, crafting system, monsters, building, multiplayer: the list doesn't stop. When you get tired of that, you can try out a texture pack to change the look, or modifications or "Mods" to change the game itself. Don't take my word for it try this out for yourself.
Some links to get you started:
Minecraft Website (To buy the game or watch the trailer)
Planet Minecraft (This is a forum where MC fans everywhere can post their work mods, text. packs, skins, etc.)
For some of my work, go here