It's basically Metroidvania meets Minecraft and is exactly as awesome as it sounds. I've yet to do a proper single-player run, but I've played through it several times online with friends and many fun shenanigans were had.
One of my buddies is like, super into it and has all sorts of videos on his Youtube channel of challenge runs, like almost beating Plantera with a copper shortsword in a ~45 or so minute endurance battle. He only lost because he didn't bring enough health potions. I'm not that obsessed, but it is a really fun game with a bunch of cool weapons and armor to craft and find, and lots of cool materials to build stuff with.