I had played the original version a while back and thought it was interesting. I bought the Alpha last night and was up until 4AM.
This shit is sick. I started out digging an underground lair and kept working until I had a little castle. I've got two mines accessible from the interior of the castle, too. I had a third but I inadvertently flooded it. I almost lost another mine to lava but I managed to plug it in time.
I was set back several times by exploding zombies. It's mostly annoying because I have to replace my front door every time it happens.