It’ll be difficult to judge this trade until we see what kind of contract Zacha ends up with. I assume the Devils traded him because they couldn’t agree on one, and they want to contend for a playoff spot this season, so they opted for a proven middle-six NHL player for a year instead of the usual 2nd or 3rd round pick for an unsigned RFA.

Boston doesn’t need Haula if Krejci and Bergeron are coming back. I guess they’re confident that those guys will take a big enough home-team discount to leave some cap space for Zacha, whom I think has been disappointing so far but maybe they think he could benefit from a change of scenery.

Through the first 12 games of the season this trade looks very much like a win-win for both teams / players, which I didn't expect at the time the trade took place.
Pretty much every Devils fan thought that Haula was gonna compete with Boqvist and Mercer for the 3C spot but that's not have thing have turned out so far.
Haula has mainly been deployed as LW playing with Hughes and Bratt, and has contributed significantly to the way this line tilts the ice against the opposition.
Hopefully his production can hit another gear but I have a hard time seeing Zacha providing close to the same value for the Devils should he have stayed with the Devils.

As for Zacha it seems like has fit in quite well.
I haven't watched the Bruins yet but his counting totals and possession metrics look solid.