FiveThirtyEight just put up an interesting article about the best and worst bullpen managers since 2000 and how valuable it is. Their measure is how well the manager does at using his best relievers for the highest leverage situations, which of course isn't everything. Still, it's an interesting read. Their conclusion is that bullpen management can swing a single win in either direction over a season. The best bullpen managers by their measure gain a win above average, and the worst lose a win below average.
I think it's mostly solid analysis at first pass. I think the biggest thing it doesn't account for is how tired vs. healthy/effective a guy is on a certain day and the effect on whether the best relievers continue to be available all year and the following years. Torre didn't lose any credit for Proctoring Proctor, for example.
Oh, and they say Farrell and Francona are both in the middle of the pack. Not top 10 or bottom 10. Gibbons is bad, Girardi is good according to them. They don't seem to acknowledge Maddon's genius...