A great story set on a grand canvas that only the Grandmaster of SF can tell. This won the Hugo Award as best trilogy that beat Lord of the Rings, yet hasn't been adapted to a movie because its story is based on various lives across hundreds of years that span the galaxy. The concept of the Ancient Roman Empire set in space would have been enough, but it's also a masterpiece of storytelling.
A great read, and such a classic. It's one of the milestones that other books of the genre are measured against. Cons: Lots of sequels and parallel novels, so if you're the type who wants the full story be prepared for an investment of time and money.