It has a weak early game and some strong competition in space strategy, but some unique features make it worth playing.
One major influence on your strategy will be your choice of race. The races have their own distinctive ship styles, but also some variation in their tech trees and in their ship drive technologies. The latter range from straightforward warp drives (travel between any two stars at a flat speed, no advantages, no disadvantages) to ships that travel very slowly between stars, but that can build gates in systems they visit that will allow almost instant travel between them (making their expansion slow, but their defense of established systems very strong). The different drive systems are an interesting way to balance the races and make them unique.
The tech trees are the most striking feature of Sword of the Stars. Not only can they have a huge effect on how you play as you improve your industrial output, or your planet terraforming, or the types of ships you can build, but they change from game to game. Apart from some basic technologies all races can access (like the techs that grant the ability to build larger ship classes), many of the "modifier" technologies may or may not be accessible to a race in a given game, or may be achieved through a different part of the tree. One game you may get easy access to powerful shields, another game you may have to go through more research to get them, and in another you may not be able to get the most powerful shields at all. You can still focus on certain types of research (favoring beam weapons over missiles, for example), there's just no guarantee that a favorite tech will be waiting for you. It's a feature that people used to 4X strategy games may love or hate (for my part, I love it). It can make it hard to have a set strategy you use with a given race each game, but it also makes the game a little more unpredictable and forces you to adapt as you research. Of course, the drawback of the random tech tree is that, combined with the impact some technologies can have on your fleet, you might find yourself far into a game and then realize that the strategy you've been pursuing is a weak one for the technologies you can access.
Another problem with the heavily tech-reliant strategy aspects of the game is that research feels very slow. Indeed, the game can be pretty dull in the early game as your ships plod between the stars looking for suitable colony worlds, and your first fleets will consist of just two or three ship types using weak, basic weapons and negligible defenses. Slow and weak is boring. If your attention span can endure those early rounds the game really picks up when you can access all three ship classes (destroyer, cruiser, and dreadnought), and have a good selection of hull types for each. As you gain new ship techs you design your own ships, assigning hull types to three sections of the ships, as well as the weapons you want them to carry. Your research can also determine how much information you get during a battle and how many ships you can field at a time. So while it's a drag getting through the early game, the midgame and beyond gives you a plethora of choices when designing your fleets, and the number of colonies you'll control by then will let you field armadas comprised of many specialized ships. You'll still have to wait several turns for fleets to travel to enemy colonies even late in the game, though, so the tempo never reaches anything I would call fast-paced.
The real-time strategy element of the battles is simple enough. For the most part you select ships, move them, and tell them to attack. You can adjust how autonomously they attack, but there aren't many special abilities to be used - most of the time you'll tell ships to attack a target or to engage at will, and then sit back to watch the results while they unload their weapon complements at the enemy. That means the outcome of combat is more dependent on your fleet composition and the technologies your ships use than your manual intervention. Unfortunately the auto-resolve feature for combat tends to lead to more casualties than you might expect in lopsided battles, so you'll often feel compelled to take control of battles even when the outcome is obvious. One nice feature that saves the RTS element from being a complete negative is the ability to zoom in close to an enemy and direct your ships to attack a particular part of the enemy ship. That can allow you to disable a particularly troublesome turret or to knock out a foe's engine so he can't maneuver. Also, ship turrets do have a limited field of fire, so turning a broadside to a target can allow you to fire some turrets at them but not others. This is especially obvious when you enable a sensor view of the combat (which shows ships as icons and will display cones of fire for a ship's guns), but for some reason you can't control ships from that interface until you've researched a particular technology. Go figure.
Diplomacy in Sword of the Stars is fairly basic, but it has some bells and whistles. You have to research the languages of the races you encounter (giving you an early advantage dealing with other factions that share your race), then you can start with basic cease fire agreements, then non-aggression pacts and alliances. To curry favor you can help defend an ally's colony or attack their enemies, or you can give them monetary or scientific aid. The hidden fanciness comes in the form of the chat interface. You can type messages to human players as normal, but with computer players (or human players who don't speak your language) you can compose messages with a menu-driven interface. So you can actually tell a computer player to attack a given system or help defend it, or ask them for money, or advise them that you plan on colonizing some newly discovered planet. You can tell them you like or dislike another player, ask them how they feel about you, or ask them to stop harassing an ally. It's surprisingly versatile, and especially nice for single-player games.
Another neat feature is the approach to multiplayer games. Turn-based strategy games can go on for a long time, which makes the ability to save and resume them later a handy feature, which Sword of the Stars does have. On top of that, however, are some more frills that make it even more appealing for multiplayer with friends. You can have a computer player replace a human player who has to leave, and that human player can take over for the computer later (even setting a password on the slot so no one else can take it). You can turn a single-player saved game into a multiplayer game, and vice-versa. If the game host leaves, other players get a copy of the saved game so someone else can host the game and resume playing. And you can set up a standalone server that will run even if all the human players leave - set up a massive game, play a while, then come back the next day and see how things went after you let the AIs take over.
The interface used to control everything in Sword of the Stars is a mixed bag. The interface is largely clean, which is good, but sometimes it can make it hard to find certain information, which is bad. Some important functions (like cycling through idle fleets) are hotkey-only, with no GUI button equivalent. Some functions I would consider fairly basic are missing entirely - I would think that in a game where a galaxy can include some 300 stars, the ability to search for one by name would be essential, but it isn't in SotS. You can double-click most messages to focus on the relevant system, and stars referenced for attack or defense by allies get markers on your screen, but other times, if an ally lays claim to a star or if you just want to get back to some system later to send a colony ship there yourself, it can take a lot of squinting and clicking to find it. The star map is 3-D, so if you have a spherical galaxy it can take a lot of zooming, panning, and rotating to check all the star names for the one you're after. The interface, then, is passable, but after two expansions and many patches it's surprising how often I found myself stymied trying to look for some basic information ("Which planets are the biggest drains on my treasury?") in capsule form, or looking for that one star out of two hundred that I wanted to revisit. The ability to type in notes and attach them to different stars is nice (and you can share them with other players to boot), but it's no replacement for some basic charts and information screens.
Graphically the game is solid but not spectacular. I wouldn't expect any problems on lower-end systems. The star map is where you spend most of your time, and it's largely just dots with lines between them. The ships are distinctive up close when you're in the combat screen, but if you zoom out they start to just look like bigger and smaller versions of each other. You won't wow anyone with eye candy, but at least the game won't hurt anyone's eyes either.
On the whole, Sword of the Stars is a game with a few flaws, but overall makes for a very fun strategy game (especially if you can pick it up at a bargain price). You just have to make it into the midgame before it starts to really shine. If you like turn-based strategy in space, and would enjoy sending custom fleets into very basic RTS battles, Sword of the Stars would make a good investment - especially if you have the patience for its uneven pace. If you prefer a more even strategic experience, or like more tactics and special abilities in your RTS battles, you might give Sword of the Stars a miss.