Anytime you can acquire the league's leading RBI producer, it't not a bad thing, but the Mets are already a bit overcrowded in the outfield (particularly from the left side with Curtis Granderson and Michael Conforto needing to play regularly). The deal is pretty much a short term and long term hedge against Yoenis Cespedes not being able to play due to injury in the short term and testing the free agent waters in the long term.
As far as who they give up -- Brandon Nimmo and two lower level prospects not yet identified. Nimmo leaving is not going to cause too much heartache. Lower level prospects may or may not as they are always a long term crap shoot.
For a team struggling to score runs, you have to like acquiring another 25/80 guy, though he's another in the mold of free swingers like Lucas Duda and Curtis Granderson. He's a career .249 hitter having his best season. At $12.5 million this year and a $13 million option for next year, he's a good baseball value.
Perhaps best of all, Travis d'Arnaud and Dilson Herrera are still on the club which means Jonathon Lucroy may still be in play....and then there's that rumored acquisition of a middle reliever.
It should be an interesting few hours leading up to the 4:00 PM deadline.