My review: Rebellion improves the game in every possible way.
The new factions are incredibly diverse, with options ranging from allying with the pirates to building multiple starbases in your grav wells to sacrificing your own population to kill nearby foes.
They've added the conspicuously-absent corvette class - each faction gets a unique corvette that really helps add flavor AND makes low-tech fleet composition more interesting than just spamming light frigs.
User interface now has race-specific skins.
The imposing Titans are a satisfying strategic option that's much less turtley than racing superweapons but has the same diplomatic and technological effect.
The new techs add a lot of depth - the sub-factions are different in deep and interesting ways.
The graphical upgrade is subtle but amounts to a much more enjoyable experience. Solar flares peel off suns, distant stars reflect off advent ships, frigates go into shadows behind titans. The improvements are particularly noticeable in combat particle effects, which look less like ships throwing sprites at each other until someone turns into a yellow ball and more like the climax of the latest Star Trek movie.
Moon planetoids were confusing at first, until I realized that Sins maps are based on abstract representations of physical space. Phase lane distance represents the time spent to travel, not actual time. The locations and distances of phase lanes can be mapped out, and whatever plane you end up fighting on will of course feel horizontal. The "true" 3D map of a sins map would be impossible to navigate - like Star Ruler but even more complex because travel is only possible along the phase lanes. So, all that to say: moons feel right. I'd like to see them better distinguished from asteroids, something I believe is currently being worked on (still in Beta 2 testing; Vasari aren't even out yet).
Still a raving Sins fanboy! I love the direction Rebellion is going. This is 10 times bigger than the previous expansions.