This is one of those rare instances in a game with such in depth progression where the gameplay itself is actually really fun and not just a grindfest. I just finished the game last night (3 stars in all and lvl 51). And I find myself starting a new game just to enjoy the challenge again.
I am especially impressed by how well thought out and balanced the skill trees are. Especially at the early stages, you can spend a little time looking at the map and decide the best way to arrange your skills to fit the scenario. On my second run through my goal will be to 3 star each stage and then their night stages in the first try without grinding additional levels.