They lost the trade itself, you always do losing the best player, but you can't ignore the implications of this trade in that it helped them secure the #1 OA pick to get Matthews.
So in some way does that mean they won the original trade getting Kessel from Boston since he was the best player involved?
Yes I know the Bruins won a Stanley Cup in 2011, however with the exception of Tyler Seguin's performance in Game 2 of their Eastern Conference Final series against Tampa Bay, in the Cup Final his only impact was 1 assist in six games played just against Vancouver.
In the 2013 Stanley Cup Final with Seguin playing a much bigger role he managed 4 assists in six games, however this time the Bruins lost that series to the Blackhawks. Plus Dougie Hamilton did not play any games in the Final that year and played 7 total playoff games prior to the Stanley Cup Final.