All of this is rather undermined by the fact that only Xbox Live Gold subscribers can currently play, with no word regarding whether non-Gold subscribers will be given the chance to jump in. Hence, to actually play this "free-to-play" game, you need to be paying a subscription fee.
Of course, this can be seen as a shallow niggle given that Xbox Live Gold offers many more features on top of simply allowing access to this one game, but it still means that Happy Wars is technically not free-to-play at all -- rather, it's a freebie for subscribers with in-app purchases thrown in.
Given that Sony's own PlayStation Plus scheme gives away dozens of games for free every year (including brand new releases), the "free" elements to Happy Wars begin to look less and less appealing.