Space Engineers

Have played 677 hours at time of writing.
I both love and am incredibly frustrated by this game. The core gameplay is about building vehicles, and the systems involved with that are very engaging. The physics have plenty of flaws, but are sufficient to make for a great experience problem solving vehicle design and piloting. Learning the basics and achieving your first milestones is very rewarding. The seamless nature of travel throughout the solar system is wonderful. I'll never forget watching the planet shrink away from me the first time I flew off into space.
But here's where I get critical. There is very little content outside of the building sandbox. Once you understand the basics, the game is trivial. There is very little survival aspect. You need only supply yourself with electricity and oxygen, which is really easy to do. There is very little in the way of violent threats in the vanilla game, and they're not really meaningful threats. There isn't much reason to actually do the work of building a ship and traveling through space. There aren't really any problems to solve or challenges to confront, without self-imposing them. Once you understand the basics, you can essentially beat the game in a couple hours.
Start on a planet, gather small amounts of a handful of resources that are easy to find and collect. Slap some thrusters on a cockpit attached to a storage block. Get up to space and collect a small amount of the couple resources that aren't available on planet. You're done. There's no *reason* to do anything else. It's pure sandbox.
However, the game has a thriving mod community which addresses this. Delve into mods, and you can get hundreds, even thousands, of hours out of this game. The engine and the vanilla sandbox are an incredible foundation for building actual gameplay on. But the entire time you are enjoying this, you will be immensely frustrated by the dev's complete negligence in addressing low-hanging fruit issues with their game that hold back what the mod community is able to do to make their sandbox into an actual game.
Examples:
The community has done an incredible job introducing a meaningful presence of NPCs into the game. Space battles are wildly impressive and fun. But it seems to be really difficult to incorporate decent AI, such that if the NPCs attack your base, they don't start by crashing themselves into it. There's mods that add new elements to the physics, such one that adds proper aerodynamics, which greatly improves the engineering aspect of the game, but completely breaks NPCs such that they will just repeatedly spawn and crash in an atmospheric environment. And there are critically lacking capabilities of the engine to enable much interaction beyond NPCs just spawning and attacking you.
Or there is the ability to mod in extra resources for the sake of adding a progression element to the game. Have your basic resources that you can gather as normal in your starting scenario, but then modded resources out in the rest of the solar system that you can use to build more advanced components, giving you an actual reason to build a ship capable of interplanetary travel. Potential for this to be incredibly fun multiplied further by the ability to add custom modded planets to your game, which are available plentifully on the workshop. But the engine imposes an arbitrary limit on the number of voxel textures, and only leaves a very small number open for custom voxels. So you'll spend those hours putting together a modded playthrough with a bunch of cool looking planets and custom resources, only to find that almost all of this custom stuff gets painted over with the basic stone texture. You can't even find those modded ores because they blend in with the rest of the rock. I sincerely doubt modern machines would be troubled by removing or raising this texture limit, but the community has been asking for it for years, and Keen won't do it.
It's truly beyond my understanding. This... program... is sincerely really cool. I just don't understand why Keen seems to *refuse* to put any effort into making it an actual game with meaningful things to do. Or more importantly, to support a community that would really love to do it for them. Even just raising the voxel texture limit would give me the ability, with the mods available on the workshop right now, to put together an experience 10x more interesting than I can right now. But I don't think it's going to change. It's a damn shame. I can only hope that their upcoming VRage3 engine will release without some of these limitations.

If you are autistic you will be obsessed

Love the game.
I plummet down to earth and immediately start gathering rocks, I use the rocks to build a base and build the base to mine the rocks so I can use the rocks to build a vehicle to go mine the rocks to bring back to the base, I build a detctor to find different rocks to build more tech to collect rocks faster. Basically just rock gathering and developing better tech to gather rocks faster to build faster and so on.

a lot of my time spent in this game involves me imagining how much fun i could be having if only i had friends

A week ago I was diagnosed with a terminal illness and the doctors have given me a year at best. I'm so glad I was able to play Space Engineers before I go. I don't know how many people will see this but I recommend to you to play this game if you do see it. Building, atmosphere, graphics. everything is masterfully crafted. If you read it, thank you and good bye.

Yes, yes, and yes. Made a best friend because of the game and sadly he has passed away but the memories that was made from this game last in the blueprints. It is a damn good game on your own but its 10X better when you play with friends.

5557 hours sow how i feel about the game