SINS really is it's own game, but some of the comparisons I've heard include the following:
-GalCiv in real time
-Homeworld meets GalCiv
-Total War in space
-Supreme Commander in Space
-Homeworld with an Empire
-[insert Classic 4X title] in real time
-Starcraft meets Galciv
GameTrailers just released a review that gives a quick overview of the gameplay:
http://www.gametrailers.com/player/31207.html