Since they may no longer be an intact quintet (and the loss of Outman makes the acronym a whole lot harder), we should examine their collective performance, with special emphasis on the last 5 weeks.
For the season:
341.2 IP 336 H/107 BB WHIP of 1.23 ER 144 for an ERA of 3.79
215/107 K/W ratio of 2/1 5.67 K/9 innings
Since May 19th:
193.1 IP 171H/47BB WHIP of 1.13 ER 69 for an ERA of 3.21
129/47 K/W ratio of 2.7/1 6.03 K/9 innings
Simply put, this is so far one of the most impressive collective performances by a young rotation in recent memory. Of course the entire season will be the more complete test, including the dog days and when some of these guys start to hit the wall in terms of their experience. I don't expect them to be able to repeat the last five weeks in the next five weeks. But the fact is that a rotation consisting of 4 first-year starters and 1 second year starter has been among the best in the entire sport for the last 5 weeks and is clearly above average for the entire season.
Seasonal ERA+ are: Braden 126, Outman 118, Cahill 112, Anderson 72, and Mazzaro at an other-worldly and small sample of 234. Only Anderson is pitching substandard, akin to Mulder most of his rookie season. Cahill, Outman and Mazzaro are, so far, approaching what Hudson, Zito and Harden were able to do in their rookie/2nd year campaigns. (Hudson was 142 in ERA+ in 1999, 113 in his first full season in 2000; Zito 172 in 2000, 125 in his first full season in 2001; Harden 100 in 2003; 114 the next year; Mulder 86 in 2000 and 126 in 2001) I said approaching because a) they're not quite at level, unless the last 5 weeks becomes the norm and b) the bar is even higher down the road, and we have no idea how their injuries or further development will pan out.
That said, so far the grand experiment does seem to be a pretty unqualified success.