It's addicting as hell once you start spelunking. In the first cave I found I've died a few times after collapses and drowning myself (lol). I spent most of my time exploring above/below and building a glass fortress w/ a garden and the beginning of a mine shaft. I bought it yesterday and played for a solid 10 hours
Even for an alpha game it's still polished enough to enjoyably play. I figured I'd buy it now before it goes to beta and the price goes up.