Let's look at a few metrics. Others have posted relievers' ERAs, but ERAs are notorious for misrepresenting relief pitchers. Let's try a couple of other things.
First, traditional stats. Bullpens are supposed to save games. How does Boston, as a team, rank regarding saves? The team has nine, tied for third in MLB. That's no problem.
But that's Paps.
Let's check holds. Now, Boston is tied for 19th in MLB with nine. That sounds bad--but AL teams get fewer holds because of the DH rule. Boston is in a three-way tie for 7th place among AL teams, right at the middle of the league.
WXRL is a measure of how many wins a pitcher, or the bullpen, is worth over a AAA pitcher or pitchers. Boston's bullpen as a group is at 1.054 wins, 15th in MLB. Again, using a more-sophisticated metric reflecting the complete bullpen, Boston is average.
Here's what I'm seeing: Papelbon is doing a great job. Combining him with everybody else, or choosing a metric avoiding his contributions, brings the team to average or less. For a contending team, that's probably inadequate.
Why isn't the bullpen thriving?
1) Mike Timlin. His ERA is astronomical, but one can make excuses for an extraordinary BABIP allowed and a high HR/FB ratio. The trouble is, even accounting for those factors he's pitched as if he were a 5.68 ERA relief pitcher (xFIP). One can say "small sample size," but Timlin's 2005-2007 stats were "lucky:" his ERA has been less than his xFIP by a bit more than a run over that whole period. Frankly, Timlin was nearing the range of a AAA pitcher in 2006-2007; another slight decline with age and injury in 2008 has left him below that level.
Sadly, Mike Timlin needs to leave. Perhaps he could be enticed into a coaching job at the same salary of his MLB contract through 2008, and he could retire...but he's got to go.
2) Manny Delcarmen. Look, I don't think that MDC is doing any worse than he did in 2007, but he's getting average vice superlative support from his fielders and he's had bad luck with fly balls leaving the park. Furthermore, his mistakes have been ill-timed: his failure to hold leads has been as costly as the bullpen work of Timlin and Tavarez combined. I see this as a case of bad luck and unrealistic expectations for Delcarmen, not a reason to trade or release him. Still, his work has hurt the team.
3) Kyle Snyder. I hope that Snyder recovers in AAA Pawtucket. He's right where he needs to be.
Besides that, Boston has a good set-up/closer duo, two capable seventh-inning guys (Aardsma and Lopez), and a veteran swing man in Julian Tavarez. Ditching Timlin for something better--my vote for "something better" being the now-well-rested Craig Hansen--along with a return to career norms for MDC--should bring the bullpen back to playoff-caliber, albeit not domination.