It's a very good game.
It is best played with four players. The expansion is cute and all, but the standard game is perfectly balanced - which is quite difficult to do, considering that each god has special (different) powers.
It usually goes like this:
0-5 games: Khorne is overpowered!
6-10 games: Khorne is underpowered!
11-15 games: Nurgle is overpowered!
16-20 games: This game is really balanced!
It doesn't have much of a Warhammer feel, because the setting is in the past (? old world refers to that I suppose).
The problem is that it takes a dedicated team in order to play CitOW. It's not newbie-friendly in a sense that if a god is missing, someone has an advantage (and by missing, I mean, not played properly). Once you learn how to play the game and develop strategies, CitOW is very rewarding and has a high replay value.
I'm afraid that while the expansion is done very well, the rat god is not able to replace any of the other four in a four-player game; it's more of a way to enjoy CitOW with five players.