As with all the Legacy projects, the layout and art are phenomenal here. That alone may be worth it.
I have a strange love affair with the family level play that Legacy encourages, but I've never actually gotten to play it - my Legacy games have not been done at that level. But Mirrors goes and makes Family level play even more awesome.
The addition of Factions along side Families is very interesting - I dig the idea of a few members from different Families being involved in The Cult of Weathertop, giving you common enemies, or avenues for spies in your midst.
Mega-projects, though, are the killer feature here. Large-scale things a family can undertake, like building cities or starting wars, that can take many ages. What I like most here is how it makes the Family play into almost a competitive resource management game.
Great work, guys.