I've been hooked since I started playing it. It's way more addictive then Minecraft.
The maps, even the small ones, are absolutely huge, there is tons to discover.
The only negative I have is it was a bit of a chore getting multiplayer to work, you need to forward the right port or it won't work. I haven't figured out yet how to get 2 pc's running multiplayer to an outside server.
I just finished my irrigation project to drain the ocean into an underground lava lake, finally managed to defeat the first boss and still feel I've barely scratched the surface after 25 hours.