David Robertson, since May 8
34.2 IP, 27 H, 20 BB, 40 K, 1.82 ERA, 1.36 WHIP, 1.764 WPA, 2.36 FIP
Boone Logan, since May 23
19.0 IP, 12 H, 6 BB, 18 K, 1.42 ERA, 0.95 WHIP, 0.494 WPA, 2.62 FIP
Joba Chamberlain, since July 28
8.0 IP, 2 H, 3 BB, 6 K, 0.00 ERA, 0.63 WHIP, 0.260 WPA, 2.83 FIP
Sergio Mitre, as Reliever
22.2 IP, 12 H, 7 BB, 13 K, 2.38 ERA, 0.84 WHIP, 0.127 WPA, 4.27 FIP
Kerry Wood, with Yankees
7.0 IP, 6 H, 4 BB, 9 K, 1.29 ERA, 1.43 WHIP, 0.134 WPA, 4.63 FIP
Robertson and Logan have been quite good since their early season woes - I'm not sure that there are many that would dispute this, either. Robertson's control isn't quite where I'd like it to be, but he's striking batters out at a tremendous clip (10.4 K/9) and has been garnering weaker contact as the year has progressed. Logan's been better, as well, since Girardi began using him as something of a lefty specialist (as opposed to cannon fodder against righties) - I really can't fault his performance in any reasonable way.
Chamberlain and Mitre are sort of difficult to deal with. I, for one, feel that Chamberlain still has it in him to be a fantastic pitcher. This may be confirmation bias, but he has been excellent over his last few outings and, despite his unsightly ERA, his strikeouts and walks have been solid throughout the season. Many Yankee fans, myself included, still dread the thought of Mitre in the rotation ... but he's done well in the bullpen. Granted, I wouldn't want him in a high-leverage situation - but I think he's done better than a mop-up type of reliever at the same time.
Wood is, in my mind, the very definition of volatile. Brilliant in 2008. Up and down in 2009. Mostly down in 2010. That being said, he's done well with the Yankees. His control's a bit off and he did allow a blast of a home run, but his stuff looks fantastic ... and being out of the closer's role (and looking for his next contract) could work wonders for Wood.
Marte's set to return soon. Aceves's set to return soon. Albaladejo looks like a completely new pitcher. And Rivera's been his brilliant self. While the bullpen's been something of a roller coaster ride this season, I cannot help but feel that a corner has been turned ... and I feel comfortable when Girardi motions for a reliever going forward.