I figured this needed its own thread so we'd stop talking about it in the Terraria thread.
It's available on Steam now as "Early Access" but for the most part it is a complete and fully functional game. It costs about $15 USD, though many people I know have simply shared the game files with each other and it works just fine that way, apparently.
The game is indeed like Terraria, though lacking the same quirky charm and atmosphere. It makes up for it with to some extent with the fact that online servers for the game are vastly more sophisticated, whereas playing Terraria online with more than a couple of people is next to impossible. Because you can travel between a nearly infinite number of planets, you don't really have to worry about the world you're on getting played out. Once you've mined everything up and explored all there is to see, you can simply pack everything up and move to a new planet (as opposed to starting a whole new game.)
There seems to be little point in building a home on a planet when you can make a mobile-home of sorts out of your starship. The amount of space you get with your starship is incredibly limited though-- there are mods you can get to fix this. The modding community for Starbound is also a lot better than that of Terraria. It might be easier to mod Starbound. I don't know. It's not something I do, but there are definitely a lot more, and much higher quality mods for Starbound.
Also like Terraria, it kind of stops being fun once you've beaten all the bosses and obtained all the best possible gear. Right now it seems the only thing most people do in online servers is hang out in the designated spawn area taking turns playing songs to one-another with their various instruments.
Starbound has this really nifty feature where you can download or create pretty much any music you want with Notepad. Go online pretty much any server with a dedicated "spawn" planet and there will be crowds of people playing music-- mostly from video games and anime. At first I thought they were MIDI files, but it turns out it is a file format called ".abc" which apparently Lord of the Rings Online uses as well, seeing how they now share a lot of the same websites for this purpose.