My take, Frank: Brisket is done when it just reaches tender, not a particular temp. Rendering begins in the 140s but it takes some time. Thickness of the brisket and amount of internal fat deposits (marbling) largely determine how long it will take coupled, of course, with cook temp.
One can certainly take a low/slow brisket to a particular temp then wrap and rest it. If the brisket is not tender when it reaches that temp, continued residual heat cooking during an extended rest can take it to tender. Many people cook briskets this way. Those that are consistently successful usually try to replicate all aspects of the cook each time, i.e., they cook using the same methods, using similarly sized and marbled meat of similar weight, etc. With practice and experience, one can judge then alter, if necessary, key aspects of the cook depending on the change in variables (meat grade, size) at play. Some briskets might be better pulled at a lower temp, e.g., some higher (this often depends on grade).
It is also possible to take a brisket to tender before wrapping and resting occur. With brisket this can be trickier because the line between excessive moisture loss and 'done' can be crossed pretty easily. Brisket's relatively thin nature (as opposed to, say, a hunk of chuck--but the same if the chuck is cut to the same thickness) and open grain structure can allow for significant moisture/rendering loss before the brisket hits tender, depending on the cut in question. Getting a handle on this and determining a target temp to pull the brisket before this occurs (and often before complete tenderness is realized), is what many do as well, again, allowing the rest to finalize tenderness.
Were a specific temp number indicative of tender, this board and others would be filled with numerous cooks extolling the ease of cooking the perfect brisket. Instead we find more people having the most problems with brisket than any other Q meat. In 'normal' low/slow brisket cooks, variations in grade, thickness, trim level (pre- and post-purchase), cooktemp, etc., can all have an effect on the success of the cook. Imo, targeting a specific temp--only--as the determination of 'done', does little to help equalize the variables.
Foil does this nicely. Foiling a brisket during cooking (in the upper 160s for a nicely marbled flat or packer, low 160s for a small, thin or overtrimmed flat) can help to significantly smooth out the potential rough spots that variations in the variables noted above can cause. It can make a target finish temp more meaningful (read: more successful) as key problems are mitigated by its use. Still, foil or not, I think a target temp is best used to determine when to first check on the brisket and get a feel for it--get a feel for where it is in terms of done. Doing so, especially if one does so consistently, can go a long way toward helping a cook achieve consistent results because one remembers--physically--what a good brisket felt like when one pulled it off, and then it becomes easier to shoot for the same feeling again. (We're pretty tactile creatures if we allow ourselves to be.)
I cook briskets at high heat (300+) pretty much as a rule now. I buy no-roll (i.e., ungraded) briskets of varying weights, thicknesses, trim, etc. Foiling a bit more than halfway through the cook (I go by time, others by internal temp) has the same affect as foiling during a low/slow cook--it mitigates problems and ameliorates outcomes--but I suggest dispensing with a therm entirely post-foiling for anything other than using it to feel the meat. At high temps in the moist environment the foil creates, internals can spike rather quickly into the 190s or low 200s but the meat won't be tender. Time is necessary for rendering to occur--though this is much quicker at high temps and, due to the foil, with significantly less moisture loss.
At Chris points out in his piece on brisket selection (see the Cooking tab), brisket can feel fork tender after it veers well toward overdone. Rightly so. But this, to me, is more of an argument for using a target internal as a point where one first checks for tender by feel rather than an argument in favor of using a specific temp to mean 'done'. One then catches tender on its way up, so to speak, rather than after the fact, or rather than going strictly by temp and perhaps pulling too soon.
