I was tempted to take this up in the rival magic thread but it didn't really belong there.
I'd say that Rival magic is a nicely self contained book that provides a great number things to bring into a game without necessarily making the game more complicated for the players. Unless you're playing one of the new traditions you don't have to deal with the mechanics.
I also see the three books before this one (Magi of Hermes, lords of men, the sundered eagle) and the next two books (The Church and Legends of Hermes) as largely working with and elaborating the material that has alredy been published.
I really have wanted something like rival magic for a while. You've got at least four sagas full of strange and potent antagonists ready to go.
That being said I think that my mythic Europe has been populated almost as completely as I'd like with magical traditions. We've got enough Ex miscelainia traditions that (baring one more left open for a player in my game with a strong concept) I'd feel that any more would be a complecation that would weaken the setting. As far as independant traditions for gifted folks go, We could use a strong arabic tradition or two to use for the order of suleman and perhaps a mongol tradition, but any more and Mythic Europe starts to look overpopulated with magical traditions.
So my take is that the traditions of Rival magic, Societates, the realms of power books and Hedge magic were a great boon to the game and to the line but the number of such traditons that might continue to benefit the line is quite small (perhaps 3 more might fit easily, barring ancient magic style forgotten traditions from ancient days that don't have to take up a share of the present gifted population)