The AAT is very slow, but it has high health and high damage to compensate. The Siege Mode ability is able to deal a lot of damage to infantry and vehicles. The AAT should not be too close to enemies as it will make it much harder to take them out. The AAT is also very effective at taking out the LAAT Gunships with its primary fire.
In the movies, the AAT is piloted by four Battle Droids. A Commander Battle Droid, a Pilot Battle Droid and two Battle Droids gunners and is able to carry six additional Battle Droids onto the battlefield while in the game it could only be piloted by a single Battle Droid.