Still in beta, just over 1 yearish(not really sure when it was first released in Alpha) old. The thing about it is, that the Graphics takes second place to the Gameplay. Even though it's old school graphically, you'll often come across some terrain that is stunningly beautiful.
In Single Player you start out in a World that is completely unique(unless you enter in a preset Code). That World only generates an area a certain distance from where you are, but as you move through the World and it generates new area, that generated part of the World becomes permanent. If you decided to just start wandering as far/long as you could wander, the total size of that World would approach approximately the size of Earth. IOWs, it is extremely unlikely you can ever see the whole world. Due to the complexity these Maps become very Memory intensive, which is why Multiplayer Servers often limit the Map size. One of my Single Player Maps is over 100mb in size and my best guess is that it is approx 30km x 30km in size.
You can do anything you want. Every bit of the World can be Dug/Mined/Moved. There is a bottom layer you can't go past and a top height you can't go past either, but that leaves a huge area in which to work. Build a modern sky scraper, Hell on Earth, or whatever.
