Empire had some interesting ideas but in my opinion failed to deliver them adequately. The rules as-is can't create a believable, medieval world. They need some work from the DM.
As an example, the population growth is so high that easily beats the modern world. You easily get 5% per year, doubling your population in 14 years. Another example is the small size of domains that also needs to be tweaked.
There are three scales: Barony, kingdom, empire, each with a different "population unit", "area unit" and "resource unit" size. That idea is innovating, it's good and it's useful. But the way the book uses this idea, the numbers that go along with the scale, the way you create and develop fiefdoms for the characters to control need work.
There is not enough versatility in "Empire" in my opinion. Whether your campaign is low magic, high magic, set in a the early medieval or renaissance, the rules are the same. There are very few random events with little imagination behind them.
There is a lack of rules for your liege and your vassals. No info on what you pay them or what they pay you.
There is a lack of rules for changing from one scale to another. If you want to go from barony scale to kingdom scale, you have to improvise.
For those interested in such things (I'm not personally), the art in the book isn't very good.
What is good in my opinion is the chapter on the "Empire campaign", that offers useful information.
The chapter on character classes in a campaign of barons and kings also has some interesting rules.
The rules of mass combat are solid, but I have seen similar in different products.
If you don't have a book with solid mass combat rules, want a few ideas and pointers on how to run a campaign where the PCs are rulers and you can easily afford this book, then buy it.
If not, then look elsewhere.