Marmol is in a tier of his own because of his insane strikeout rates. If he can improve on his control (a lot) from last year, he can be one of the best relievers in the game. If he can't, he's at risk of being passed by the Tier2 guys.
Carlos Marmol: Extreme fly ball pitcher, with terrible control, and incredible strikeout rates.
Keeping down the walks will be key for Marmol to be successful. Walks and extreme fly ball tendencies (home runs) is a very dangerous combination.
These guys are all fairly interchangeable. Any of them could outperform Marmol with some big strides (or continued control problems from Marmol). None of these guys have had consistent numbers, they all have some risk
Sean Marshall: Ground ball Pitcher, with somewhat poor control, but descent strikeouts. If he can lower his Walks, he could be very effective, if it doesn't, he'll continue to be average, should perform very similar to Grabow, whichever of the two can keep their walks in check, will prevail as the better pitcher
John Grabow: Ground ball pitcher, with poor control, and descent strikeouts. I'm noticing a pattern here, like most of the other cubs relievers, he needs to keep down the walks. His groundball rate has also been on a downward trend the last couple years, something Grabow should hope to correct (or he'll be giving up more home runs).
Jeff Gray: Groundball pitcher with low/moderate strikeouts, and descent control. He showed very good control in 2009, if he can continue to limit the walks, he should be successful. He has the potential to be one of the Cubs better relief pitcher's this year.
The guys in this tier have some potential, but most likely still developing, and may need another year. There's a good chance either of these guys end up back in the minors
Esmailin Caridad: Slight flyball pitcher with questionable control, and descent strikeout rate. Caridad should be an interesting guy to watch, he was extremely good in his 19.1 innings in 2009 (5.67 K/BB), but his minor league numbers don't seem to imply that he'll continue that. If he can keep the walks down, he could be very good.
Jeff Samardzija: Slight groundball pitcher, with terrible control, and low to moderate strikeouts. He'll need to improve his strikeouts, and lower his walk rate to be effective.
I don't see these guys succeeding, most likely they will be demoted and replaced at some point.
James Russel: Flyball pitcher with good control, but low strikeouts. He doesn't strike out enough people to compensate for his flyball tenancies, he won't be anything special.
Justin Berg: Extreme ground ball pitcher, with terrible control, and terrible strikeouts. Nothing to see here, he won't last long.