I already have a cube and I am anxiouslly awaiting the release of MP. I think the FPS influences and the fact that it's not made by Nintendo has given this game a bad reputation with some of the "hardcore" (For lack of a better word) gaming crowd. But look at it this way, Nintendo just recentlly bought out and now owns Retro, and the game is NOT going to be a traditional key fetching, switch flipping FPS. It's more about exploring, which has been a staple of the Metroid franchise since day one.
Getting back to the topic, I think this game will be somewhat of a system seller, but nowhere near as much as Mario Sunshine and the new Zelda game.
