The game is really what you make it.
At first you have a lot of fun just mining and staying alive and getting a basic shelter. Then you make your first fort. After that it's up to you.
I played the game non-stop for a whole day then the next day I got bored. A couple of days later I watched "X's Adventures in Minecraft"
on youtube and it gave me a ton of ideas on what kind of stuff I can get up to which meant many more hours of fun.
On top of that the game is gonna get more features in the future and be even more awesome. Here's an example: