Summary: bought it based on customer reviews and the fact that it won game of the year. I love the RTS genre and this game is one of the best I've played. There are no "missions" in the treaditional sense, but there are so many variations of maps and styles of play (including for the computer AI) that you will never get bored.
Summary: This is a great game that allows for epic campaigns and large scale battles. Starting with a single planet and few ships you progress through the tech tree developing tech to colonize different planets and larger more powerful ships. Due to changing the size of the "maps" by altering the number of planets, availability of resources and ship population limit just to name a few. I have never played this game online so I cannot comment on the aspect of the game play.
Summary: I bought this game by accident when it first came out. If you are a fan of the Master of Orion series, definitely get this game. It is by far, the best space strategy turn based game I've seen in a long, long time. There are lots of addons for it and features have been added since the original (along with two pay for addon packs that are very cheap to buy). GREAT GAME!
Summary: Sins of a Solar Empire is a highly addicting and wonderfully complex game. Not only is it an excellent example of a real-time space/strategy sim, it's system requirements are highly reasonable for a game of it's caliber.
Summary: The one great thing about this game is the mods. Be sure to visit the main game site and its forums to get info on the best mods and expansions. I got the game for cheap and the expansions are cheap. This game has tons of gameplay time and its a great buy. Impulse is also a wonderful service. Get it!