You can think of Flatspace II as a sort of shmuppy Elite by way of NetHack. Like NetHack, the universe is randomly generated each time you start a new game; like shmups, starship combat is fast and intense; like Elite, you're a starship captain exploring a huge universe--and there are a whole slew of different roles you can take (trader, mercenary, bounty hunter, assassin, police officer, or scavenger).
Flatspace II is a space trading, exploration, and combat game. Initially, you begin with a small starship with limited capacities, and have to work your way up by earning money and purchasing better equipment for your ship, and later on new and larger ships. Money can be earned in a wide variety of ways: trading the many commodities available in the Flatspace universe, performing missions (which are many of the Fedex variety), performing assassinations, tracking down criminals, mining asteroids, and so on.
Flatspace II has no real 'win' condition, but you can set yourself any number of goals--becoming rich, getting in good with one of the game's guilds, destroying the scarrids (alien enemies), becoming the most feared pirate in space, and so on. There's no real story, but depending on what goals you choose to pursue and how you play, the experience of the game can vary a lot--and when you start a new game, a universe is randomly generated, so there's a high degree of repeat playability.
Combat is somewhat "shmuppy", that is, reminiscent of space shoot-em-up games going all the way back to Spacewar. However, while intense combat can occur in Flatspace II, it's not the sole emphasis of the game, so you'll see a smaller variety of weapons, ship types, and enemies than in some other games. Because of the sheer variety of different activities the game supports, it has the potential to keep you engaged for quite a long time.
If Flatspace II has a flaw, it's in its lack of a tutorial mode; the controls do take a while to figure out, and some experimentation is required before you learn the ins and outs of dealing with space stations, finding and performing missions, and so on. As a result, a read-through of the manual is highly advised (it's not too long) before you start. And at times, you may find it advisable to take some notes--e.g., the game doesn't track the purchase price for goods you bought, and while you can assume that, say, taking food from a farm to a trading station is a good idea, it's not a bad idea to track on your purchase prices, so you know you're making a profit. Similarly, if you fed a low-cost provider of a good that's in high demand someplace else, you'll want to remember what sector its in.