The galaxy is trapped in an endless cycle of extinction. Every 50,000 years an ancient machine race invades and with ruthless efficiency wipes out all advanced organic civilization. They leave behind only the scattered ruins of technology, destroying all evidence of their own existence.
A good story, but more archetypical. Generally everyone besides Ashley Williams has depth in character, though.
The shooter/RPG hybrid gameplay, however, is less entertaining than it is frustrating. A soldier class Shepard can barely hold a sniper rifle straight, while he can hold a shooting assault rifle more steady than if it were sitting on the ground (even though he's supposed to be able to use either weapon proficiently).
Mainly, I only recommend this so to get the save-game import. It may be a good game, but its age is showing. I enjoyed playing it, but yeah... I don't expect to be playing through again anytime desperately soon.