In order to be tender, a brisket must be cooked to an internal temperature of 180-205°. The reason for this, according to McGee, is that the conversion of collagen to gelatin doesn't even begin until meat reaches an internal temperature of 140°F, and is most efficient as internal temps approach 212°F. "Low and slow" barbecuing at 225-250°F is ideal to facilitate this conversion, providing gentle heat over many hours, allowing the collagen to make its transition into gelatin. While some moisture will be driven out of the brisket as it reaches these high internal temps, the gelatin makes up for it and keeps the meat moist. Of course, the fat layer on the brisket also helps maintain moisture in the meat during cooking, as well.
Barbecue experts with years of cooking experience say a brisket is done when it's "fork tender", meaning that a fork or a probe thermometer goes in and out of the meat with little resistance. The problem is that both a properly cooked brisket and an overcooked one with both be "fork tender". For most folks like you and me, it's best to rely on internal meat temperature to determine when a brisket is properly cooked.
What most everyone does agree on is that a properly cooked slice of brisket should pull apart easily, while still maintaining good texture. An overcooked slice will fall apart when picked up.
