I looked at this for a couple of nights and am still in two minds about it.|
The demo is clearly bad. Even before talking to owners of the full game, the demo is just poorly designed, buggy and fails to sell the game. If you deviate from what the demo wants you to do in it's precise order it seems to make it impossible to advance the tutorial further. It doesn't really explain the strengths of the game - as a building game it seems bad, tedious controls to build unattractive things for reasons that aren't clear. As a resource gathering game it has some pluses on Minecraft by having such a huge variety of things to collect, but it's unlcear that you can do anything interesting with them once you have, because the building isn't interesting.
Where the game might excel is as a metroidvania fighting / exploration thing - which people who own it have confirmed - and this is not indicated very well by the demo. You don't collect experience, which is always an enjoyable part of mindless killing, and the demo doesn't suggest there is a goal to the game.
You could argue that Minecraft is the same, except that MC is obviously a sandbox building game primarily with some other things tacked on - and given the sheer depth of some of those "tacked on" elements, redstone, music etc! -
The demo for Terraria makes you focus on building then makes no attempt to explain what you would even get from purchasing the full game. It's a weird environment that I'm told isn't indicative of the full game at all.