While I know people will disagree, for these devices to be considered successful they probably have to sell to a level where it is clear that these devices have a wide appeal; and that their sales are not driven by a single popular game. After all, just because Guitar Hero 3 sold 15 Million units doesn't mean that people see a plastic guitar as a viable alternative for a controller outside of music games.
With that said, I think that anything less than being bought by 10 Million console owners would be a clear failure (with less than 5 Million being an epic failure) and anything more than 20 Million console owners buying into these devices would be a clear success. The middle-ground (10 to 20 Million) is an area where the case could be made either way, and it really depends on what games are created for the device and how well they’re received and how well they sell.
