Pierce has looked really sharp in the first two games. His dunk against Charlotte ws a reminder that his athleticism is in almost full effect. Of course he is not the Paul Pierce from the 2002 Playoffs, but he still has lots of his old self left.
As Celtic already said, Pierce is a smart player who makes very good choices on positioning and is still one of the best in footwork skills. And as we have seen in the preseason and the first two games, he can still drive like his old self.
I would give him 3 more years at this level, where he only declines marginally in each season. After that, there could be a huge dropoff, but he will be a sixth man at the very least, and contribute a lot (like Grant Hill, Reggie Miller, etc).