If you live a life where you already don't have any friends, have no hope of ever having a girlfriend, and have given up on the idea of healthy productivity in favor of being a waste of talent then I would say this is the game for you. Otherwise you best steer clear for fear of tripping and falling down a rabbit hole from whence your real life will never return. Im only slightly joking.
There are two games I have ever played that I truly love on one hand and truly regret on the other. The first is Dark Souls, which drained away my entire year of 2011. The second, is this game and its predecessor, Terran Conflict. I have played X3 for probably 1000 hours and it was a major contributing factor to my college GPA dropping seriously about 3 full tenths of a point and leading to a few failed classes that I had to stay an extra semester to retake. There were other factors too dont get me wrong, but seriously if you don't have a healthy lifestyle to fall back on and you are a sucker for space sim type games this game might hook you and never let go until one day you stumble out of your apartment, blinded by the sun, realizing you havent shaved in 8 months and have lost 30 pounds and arent sure what year it is or what happened to your life or any of the people you used to know. It is simultaneously the best and most dangerous thing you can buy for twenty dollars. Whats sadder still is my hard drive failed and so I literally have nothing to show for at least 800 hours of my life. Holy Sh!t.
I tell you all this because theres no way to sum up what you can actually do in the game in any reasonable amount of words, but heres a few useful bits I have divided rather arbitrarily into 3 good points and 3 not as good points.
1: Think of EVE Online, except its a single-player sandbox with about 200+ beautiful sectors populated by about 8 races, each with their own ships, technologies, stations, food supply chains, goods, etc.
2: You also can actually fly your ship (theres hundreds to choose from, some you'll have to capture but seriously you'll find something you love) itself with wasd or a joystick and dogfight unlike in EVE.
3: You can basically do whatever you want from escorting passengers or assassinating criminals to mining manually or by setting up individual stations, fill holes in the economy to help AI fractions or build self-sustaining war fleets or illegal drug empires and supply chains that exist only in a single sector or stretch across the universe via automated trade fleets operating out of closed-loop complexes. You can of course be a pirate too, ejecting marines out of you cargo hold and watch them jet-pack onto a ship and cut their way through the hull and kill everything inside while you delicately keep the ships shields down on the outside (otherwise your poor meat-puppets will fry and you'll be pissed cause training them is time sucking and expensive). You can command huge fleets by piloting one ship, waiting till its on its last leg, ejecting into space, and teleporting into the cockpit of another one within 15km of you and keep hopping around as needed, some engagements can go on for a few hours if you want them too. Heck you can even be a bloody day trader using the stock market.
The less good parts (and things to take note of to help you).
4: The interface takes some getting used to as its not all that intuitive but theres so many options and key bindings that I can see why making one would be hard. You also need to remember that software upgrades and racial notoriety are the keys usually to success in the game. You also need to pay attention to what areas need what goods and set up satellite networks to watch prices and supplies.
5: The vanilla universe can be a bit dry though because the AI factions and even the pirates arent always aggressive enough towards you or each other. The economy can also kind of sputter out in certain areas if you don't help it out. That said, I highly recommend you go to the Egosoft forumsd an download either the X-Tended Mod or the XRM mod as that will solve this problem. You wont be able to get achievements but who cares about that anyway? It also might disable the plot but the plot missions are soo long and absurd for such little story in return that most players dont bother and just create thier own roleplay story as the play in the sandbox.
6: I find managing large fleets to be difficult as there just arent enough options available for commanding large groups of them in meaningful ways. The combination of SETA and autopilot can make things either crash into staions occasionally or make AI ships circle each other forever without ever being able to accurately hit each other. Little bugs here and there like jumping through a gate in a large ship and accidentally ramming into a smaller ship on the other side and killing it, so the po-po comes after you. Large trade fleets are tough to cause you rely so much on remote piloting by software commands and it becomes tough to keep track of them all and rememebring what routes you have going where, what sectors to avoid because of the Xenon and whatnot. Again scripts and mods can fix a lot of this stuff though and I HIGHLY recommend modding the game as any issue you have with it or any added feature you would want someone has though of on Egosoft forums and has made a script for it. The community is really helpful and dedicated, though it has shrunk some because of the unforgivable mess known as X;Rebirth I should also mention theres a few exploits in the game too like nVidium mining and stock market abuse.
I hope you find this review to be of some use to you. Be aware of what happened to me and tread carefully. Its a great game for those with patience, intellect, and no hope of success in the real world. It is probably a grave risk for anyone else. If you do take the plunge on an X game though, this is the one to get.