Brisket is done when it's done. My first one was tough, but that's because I was cooking for temperature. I had one other come off "tough" and that was my first and only attempt at hot and fast brisket.
I've made somewhere around 100 briskets and they always come off tender (with the aformentioned exceptions)
I cook around 225 - 250 for APPROX 1-1.5 hrs a lb
A couple of things I've picked up.
1) ALWAYS rub your meat.
2) after the first 2-3 hours your brisket will not take on any more smoke.
3) foil after the first 3 (or so) hours and probe till done. You WILL know when this is. My magic number for most of mine is 210 degrees (in the center of the thickest part), HOWEVER this will vary and is ONLY a rule of thumb, do NOT pull until done, as this temp WILL vary due to many different factors, such as : fat and marbling content of meat, humidity in cooker, etc...
4) Always let your meat rest. I understand what others are saying above, how you want to cool it down quickly if your overdone, but to me "overdone" means it's falling apart. Pull it before then. The contrast between not done, and done is amazing, and you will notice the difference. Once you get this down, you'll want to pull just before "butter" so you don't have this problem.
Play around with injections, they will add a level of moisture and flavor to your brisket you probably didn't know was possible. Butchers is awesome! Get some. Try it, play with it, make it your own.
Most of all... don't give up!