In a surprise release, the Blast Ball mode of Metroid Prime Federation Force is now available on the eShop worldwide. (though I think there might be differences between the NA and Euro versions).
I'm played it for some hours, it felt rough at first but after a while the game started to "click" for me.
The online appears to work well, it's possible to spectate when waiting in a lobby and even to have another player join mid game if an existing one gets disconnected (can't think of any Nintendo game that did that).
The Challenge mode where you have to go up against 5 different AI teams though is very hard. Managed to barely win the 5th round with a human team.