The strategy scales nicely with player counts. In fact, it gets possibly even more interesting, since there's a greater chance that you'll have an opponent who will share role actions with you (which means you don't have to bloat your deck with as many rubbish cards as long as they reliably pick the roles you want to be following; and at the same time, you can wind up with larger hands coming around if you're *not* sharing roles with everyone. I find it becomes a bit like a 3-player Duke game, where things balance out nicely if two players share one strategy and the third goes the opposite direction.
The expansion is also pretty great. It adds a few things, including technologies that you can pay for with fighters (and they actually differentiate the different kinds of ships, too), "peace treaties" that get you +1 VP if you dissent on a Warfare role, and scenarios where everyone starts with a pre-built deck (which can range from "two super-powerful techs and lots of junk" to "use ALL the Politics!") and a particular planet, which puts a bit of a different spin on it. I wouldn't say it's essential, but it does add a huge amount without doing anything too detrimental to the game.
