It's free 'now' but you can buy it as well. Like Minecraft you could buy it when it was 'younger' before it got more expensive, but more immersive.
Minecraft has a ton more content (obviously) and Starmade does use the whole 1m 'block' system. As they continue to add, you will likely find similar blocks that can be found in Minecraft, but StarMade is primarily space and will largely support space stations, planets and space ships. Now.. if you've ever played Freelancer... or Freespace, or any other space shooter, just imagine what you could create.
Everything in the video is a year old and was in it the time the video came out. It was actually made along with a LOT of minecraft players and servers.
[media]https://www.youtube.com/watch?v=Os1-m4RJCkM