According to ESRB, this game contains: Blood, Violence
Parents won't have any questionable language or sexual situations to worry about, but the game revolves around destroying vaguely humanoid robots. I think labeling the game with an M was a little uncalled for since there are no humans in the game, and although there are a metric ton of guns, swords, missile launchers and other such weaponry, the violence is not gory or even bloody. It might be a little on the difficult/boring side for kids, but otherwise it's very unrealistic violence. After all, I doubt most kids have a mortar launcher installed in their chests.
Action gamers should be cautious. The fighting can be intense and fast-paced, but the combat takes place in rooms and hallways, not exactly the most breathtaking of settings. Besides fighting other androids and customizing your own, there's nothing to do, so don't expect a more varied type of adventure.
Armored Core fans would be very wise to check into Rengoku, at least until AC: Formula Front gets released. It's not exactly the same as building a Core from the ground up, but the weapon selection is unbelievable and leveling-up the android is every bit as addictive.
Deaf and Hard of Hearing gamers will have no problems. Communication is relayed through text, and no audio cues are necessary since all vital information is displayed onscreen. It's completely accessible.