A fascinating article for baseball geeks at Hardball Times: http://www.hardballtimes.com/main/article/ranking-the-relievers/
They rank relievers according to win probability change from entrance to exit, assuming their team scores at an 'average' rate. By this measure (and probably many others), the A's had a bad bullpen. But the surprise is that Justin Duke gets ranked above Octavio Dotel in value.
Food for thought.