The Cardinals bullpen has been quite good this year. They are in the top half of MLB in terms of ERA and #7 if you evaluate them on their peripherals with FIP. The other notable item in relation to the bullpen is that they haven't really been stressed to date. They rank 5th in innings pitched with just 80.2 innings to date. The Cardinals starters are doing their part to make sure that the relievers aren't over-taxed.
Mitchell Boggs seems to have taken the biggest leap forward going from useful reliever to legitimate relief ace. Jason Motte has continued to be a steady hand at the back end of the pen with a double digit strikeout rate. Both have featured excellent control. Add to that a somewhat wild but still effective Fernando Salas and left hander Marc Rzepczynski who is sporting a 4:1 K:BB ratio and Mike Matheny has four relievers to rely on in high leverage situations.
While Kyle McClellan and Victor Marte haven't been elite in terms of performance, they've been acceptable middle relievers. Both had mediocre strikeout rates and good walk rates. Marte has been bit by some untimely homeruns.
And then there is J.C. Romero with a 10.50 ERA in 6 innings on the back of 2 homeruns. He's the one guy who seems, to me, like he's pitched worse than his peripherals would suggest.
To date, Matheny has done well at picking his relievers. The leverage index for relievers (a measure of how critical of a situation they are entering when called upon) adheres loosely with the above statistics. Motte, Boggs, Salas and Rzepczynski are all being called upon in the more critical situations. Victor Marte is the backup plan while McClellan and Romero appear to be the clear mop-up guys.
Now, if we can just get Matheny to stop calling for the bunt. . .