Idolm@ster, aside from the fact that the game will advance text on you automatically (and only automatically), isn't that hard a read/listen compared to other things. You can pause the game and try to read the text, even during timed choices, if you can't read that fast and couldn't pick up what they were saying. If you're comfortable with casual speak, then the game isn't that hard to understand for the most part.
However, if you're coming from knowing nothing at all...
... I won't pull my punches: it takes a few years to get things down if you take your Japanese casually, so all most people would probably get starting out is a few words here and there, I'd think. Video games are good for your Japanese, I can say, but to really get anything out of it, you need to know grammar pretty well.