I just finished my first single player game, the very first "campaign" option, and it took me a little over 9 hours, mostly because I learned how to play as I went along, and also because I was very methodical about how I waged my war. (I did all four tutorials first, and they helped a lot. I think I only cracked the manual a couple of times during this first game.) Nine hours well spent, if you ask me (day off today lol), although the end game leaves something to be desired. If you are expecting a gratifying Civ type end game you will be dissapointed; there are some graphs of your performance charted against your antagonist (but not against the pirates, bummer) but that's literally IT. Poof, game over man...
..WHICH IS OK since everything about this game (from the setup, to the online component) seems geared towards multiplayer, with the single player games geared to get you familiar with how to play. No sweat, and I look forward to playing online eventually, but in the mean time I'm a little disappointed that all of the single player games will end in this fashion. Like I said, no sweat, it's still a lot of fun.
I gotta say that the voice-overs just killed me. Oh yeah, Stewie for the robot ships and Keanu Reeves for the siege ships (I was randomly given the TEC side, haven't played the others yet). I caught references to Homeworld aplenty which happens to be a favorite game of mine; the 3D aspect isn't as well implemented here (it's really played on a 2d plain but shown in 3D) but still is some nice eye-candy. To give you an idea, in Homeworld, the 3D aspect really mattered as far as game play and where you placed your ships; in this game, you will be playing solely in a top-down view while the game randomly positions your ships slightly above and below the horizontal plane. You can zoom in and rotate through three dimensions but it's not quite the same. I'm not bagging on SINS here, just pointing out the differences.
This does not affect the gameplay at all, since as I said before, I was able to pick up the way of things quite easily and adapt to this style of play. I liked the fact that the enemy AI would try to retreat if severely outnumbered; later in the game I was able to utilize the phase jump inhibitors to my advantage and beat the crap out of the retreating fleet; it felt good to watch them explode as they tried to flee. On the flip side, they offered an acceptable amount of challenge until near the very end when I had a fleet of 6 capital ships and dozens of frigates and cruisers and was able to take (and hold) planet after planet. Early on those pirates were indeed a PITA until I learned to place multiple stationary Gauss guns in my orbits. All in all, for my first game, there seemed to be a pretty decent balance between challenge and fun. Your mileage may vary however.
At the end of the day, I really like this game; I wasn't quite expecting this deep level of game play (coming from Gal Civ) and was pleasantly suprised with how all the mechanics meshed together. I admit that I'm a fan of strategy space games and that I hold the original Homeworld as the gold standard (HW2 kinda dissapointed me). I really really hope that the online component of SINS is as enjoyable as that of Homeworld was, and from what I've seen I don't think I'll be dissapointed. But of course if I am, I'll be sure to spout off about it here.

Good game Stardock! Cheers~