I've tried Wal Mart's select briskets a few times, but they never turn out quite right. The flat always ends up dry and chewy. I've tried internal temps from 185 to 203 and everything in between, but the result was the same. The last one I did still wasn't probe tender at 203 after 14+ hours at around 230 - 250. I thought about leaving it on, but went ahead and pulled it. I'm glad I did, as it was quite dry. If I had left in on longer it probably wouldn't have been fit to eat.
I'm using a COS with a water pan, starting the fire with charcoal and then mainly using sticks once the fire gets going. It's a pain to keep temps steady, but I usually manage to keep it mostly between 225 and 260. I've used it to cook choice briskets from other vendors, and they turned out fantastic. So I'm blaming the cheap selects instead of the cook.