Ben Cherington's record as a GM is difficult to assess. For every point there's one or more counterpoints.
Point: He won a championship.
Counterpoint: 3 last place finishes.
Point: He made 3 albatross signings in the same year (Pablo, Hanley, Castillo).
Counterpoint: At least one of those was forced on him by Lucchino.
Point: He left the team in great shape for the next GM.
Counterpoints: He left those albatross contracts. He left very little in major league ready pitching. Some of what he left, like Betts, was left to him by the previous GM.
Adding to all that is that he has never had any other GM job you could use for further evaluation.