Be warned - this game can get insane.
As the title suggests this game is a fleet (unit) management game, mixed in with a x4 type of experience. The scale of the game opens up suddenly and can get to the point where it just seems overwhelming, however if you stay with it and learn how it works you'll find a decent game.
I'd recommend taking this game slowly, even if the AI doesn't really want you to. You will die several times as you discover the past ways to link units together and arrange your resource collection and defending units to be most effective - that's just the nature of the game. As such this isn't really the ideal game for a beginner to x4 to step in, yet it might just be one of the most rewarding.
The depth in potential is deep and the 3 expansion packs have been done in the 'right way' adding more content to the game and improving gameplay. Thankfully the developers regularly patch AI War: Fleet Command too, which would suggest they're aware of their community, which for me is a major part of buying into a game.
I know some people aren't too keen on AI War due to the rather dated UI and graphics, but I think that's part of the charm. The lack of flashy graphics allows for resources and time to have been spent on the complex nature of the game's actual 'AI'. So, considering the game is called 'AI War', it's almost essential that the game itself has a top level AI behind the scenes making all of this work.
At times this can feel like the enemy are being hyper aggressive, but in truth they're learning about you the player. They're not just reacting to whatever race you play, they learn how you use that race and how you setup. Again this might not be ideal for a new comer, however it can help too, as sometimes they'll go 'easy' on you... for a little bit.
Overall AI War: Fleet Command is a very indepth game that doesn't seem to get much fanfare amonst the Steam Community. If you're a fan of x4 games then this is the game for you. If you can look past the very basic graphics there is a very deep experience waiting for you, perhaps one of the best experiences you'll ever have.
The added bonus is that this game is often VERY cheap on Steam during the Steam sales.