Great Standalone for first time SOASE players, Xpac for SOASE vets
http://www.gamespot.com/sins-of-a-solar-empire/user-reviews/547266/
Rebellion is basically more of the same with a few updated tweaks on the engine, new sub factions within the different races and new classes of starships.
If you havent played SOASE before this is definitely the pack to get. Its a standalone product that basically includes the new material plus all of the previous material from the other xpacs.
As a player of SOASE since vanilla, the biggest drawback here is if you already owned the original game and all the xpacs, Rebellion seems a bit over priced for what you are getting which is really mostly DLC level of content and a few new ships.
Would have been nice if they had given previous owners the engine update as a patch and maybe offered Rebellion as a xpac for say $14.99 to players that already owned all the other versions of the game.
The other downer is the game still doesnt have a single player campaign which the game could really use to attract more players and flesh out the actual story more.
All in all its still the best 4xRTS currently out and is wrth checking out if you have never played it before.