Quoting: DoctorBreakfast
According to Pierre LeBrun, Pastrnak's first asking price was at least 8 million. So considering that Sweeney got him to bypass the 7 million range altogether and got him to sign for 6.667, this is a fantastic deal. He probably would've liked to ink him for 8 years rather than 6, but it looks like he sacrificed term for the lower AAV.
That's typically how negotiations with players with limited leverage goes. You shoot extremely high, such as 8,000,000 AAV when you are comfortable with around 6,500,000 AAV. This can be applied in any sort of negations.
In terms of advanced analytics, Pastrnak is similar to Vladimir Tarasenko. However, Tarasenko got paid more and had a longer length, and is widely viewed as a top-5 winger in the NHL.
In terms of the market, two parties entered and feel that they are better off from when they entered the negotiations. WE can speculate whether or not the deal is a steal, but both parties, David Pastrnak and the Boston Bruins feel better off.
I personally feel that the deal is in favor of David Pastrnak, specifically the long term. He will be able to command around a 8+ per term, and that will increase if the CBA is re-negotatied and he can command as much as the market is worth, and there are a hell of a lot of teams that want a 65+ point guy who can snipe and be relied on at will.