Nearly Five Years and over 5,000 hours since I bought the game back in August of 2014, and Space Engineers is still the most played game in my library. In my tenure playing Space Engineers I have had many enjoyable moments, and though I've had my fair share of unpleasant experiences with the game, there is always something that draws me back to its infinite skies.

Core Mechanics:
The game is easy enough to pick up and play, but takes time to understand and make use of its many mechanics. You are given a wide variety of survival starts, or you can play creative mode. Its core mechanics are pretty simple. You start with a small ship with limited supplies, you need to mine ores and refine them into usable ingots which can then be turned into components which can be used to build ships and bases. Each "block" requires components, each component requires ingots.

Gameplay:
Singleplayer survival is a lot of fun at the start, but after awhile becomes boring with nothing to do and nobody to fight. This can be remedied by going into multiplayer, or adding mods to your game. In singleplayer your only goal is to build the ships and bases you want to build while capturing the Space Pirate ships that fly near you. Conflict in singleplayer is limited, but can be a lot of fun with a big enough enemy (ie. Pirate HQ's.)

Multiplayer:
This game really shines in multiplayer. With hundreds of dedicated servers to join there is always a friendly community willing to play with you. Don't want to join a large group? Then grab a few of your friends and play together through a singleplayer world. With people around there is always a helping hand, or a threat to defeat. Be warned though, muliplayer is still quite unstable at certain times.

Mods:
Mods are really what make the game playable for so long, with such a large modding community there are always new and helpful mods for you to explore, or exploit. Many mods seek to add new gameplay mechanics, others look to add new blocks. With thousands of mods on the workshop, there is sure to be a mod for everyone.

DLC:
Since Space Engineers has been released form early access Keen Software House has been making DLC to go along with their major updates, many people see this as a bad thing. This is simply not true. Video games take a lot of time and funding to develop. The DLC blocks don't add anything that provides advantages for players, just cosmetic items. DLC may only become an issue *if* Keen puts huge game features as purchasable DLC (such as planets.)

TL;DR
While Space Engineers is an amazing game, it is not for everyone. You have to be willing to learn about its many features and grind for materials even when you'd rather be building your ship. Though its world is infinite there is little to explore other than planets. Singleplayer is fun, but when you add friends is becomes so much better.
I would highly recommend Space Engineers to anyone who is looking for a fun Science Fiction game with lots of flexibility.