Endless Space is definitely a good game, and worth playing if you like 4X games at all. Ultimately though, the game was a bit bland gameplay-wise; the setting, factions and worldbuilding are very good, but battles aren't very interactive, sometimes seem to come down to a game of rock-paper-scissors, and there isn't much complexity to them.
This expansion helps, adding more depth to combat and a lore-based win condition that fits better with the lore than plain old conquest. But ultimately the gameplay still feels kind of anemic, though it's fun and you can certainly spend several full days on it.
Ultimately though, Endless Space and its expansion was really more of a prelude to the better (and fairly different) Endless Legend. Endless Space proved the devs could make a good, full-featured game, but it seems like they quickly moved on to their next, more ambitious project. Still, the game is a worthwhile purchase, and its expansion is cheap.