This season, he'll most likely have his ups and downs. He'll explode at times and look brilliant, but there will also be games when a savvy veteran defender will eat him for lunch and his numbers will look awful. Play him when you like the matchup he'll be facing. Teams that have trouble when more than two receivers come at them will provide him with his best days.
Long-term, I feel he has the talent to be among the top WRs not only for the Patriots, but in the whole league. Talent alone won't take him there, however, so he unfortunately can't be considered a sure thing in keeper leagues. But if the Pats don't push him too much too soon, and if Branch himself has the necessary work ethic, he could be pretty special.