A recent Fangraphs story broke out the BB/9, K/9, GB%, and xFIP for starters 1 through 4 for 2010. While I'm not going to comment on the methods used by the article (which actually aren't very clear), I was interested to see where the enigmatic, low-K/9 Sean O'Sullivan stood according to this metric (for his 2010 season).
His BB/9 was 3.33, which puts him near the bottom of the #3 Starter tier. K/9 was 4.63, which puts him basically right in the middle of the #4 Starter tier. His 2010 GB rate was 40.8%, which puts him near the bottom of the #3 Starter tier. Sully's xFIP of 5.19 ended up at the middle of the #4 Starter tier, which seemed low to me since it is based on the previous 3 factors, but maybe that speaks to the relative importance of strikeouts.
At least for me--since I don't expect much out of SO'S--a so-called #4 starter is perfectly fine for a young, low-cost pitcher. Maybe his pitching approach matures and he gets more groundballs and fewer walks. Maybe not. But at least it looks like there's a chance he doesn't suck.