In addition to the increased duration (15, 20, 35), the extra strength is around 25% at rank 2, and 50% at rank 3 I believe. The bonuses are not kept after the effects wear off; however, if the unit was injured at the time of being controlled, the 'bonus health' will take the place of any missing health, effectively healing the unit partially.
Furthermore, it can be a great accessory to have when fighting foes that will attempt to board you, as the Mind Control can be used on boarders to turn them against each other, though the controlled enemy will take up one of your friendly slots in the room, and you cannot directly command them. Be wary, as this temporarily makes them an AI Crew member for your side; lacking an opponent to fight, they will go through your ship - blast doors permitting them - to rooms that require attention, such as damaged or unmanned systems. They will make repairs until the Mind Control wears off, but will turn aggressive as soon as it ends.
Also, it can be great when attempting to disable systems, such as shields or weapons, since you can take control of the unit repairing them, and they will attack it instead. Plus, they will fight anyone the enemy sends to repair the system in their stead, further preventing repair. Combined with Hacking systems, you can lock the room and allow the Controlled Target more time to destroy the system.