AtLarge just posted some stats on the recent open 9Ball tourney. The most interesting datum, imo, is the finding:
He won 63% (274 of 435) of the games in which he broke legally, made at least one ball, and did not foul (successful breaks).
Need to take this with a grain of salt, though. In a winner breaks event, the best players break a lot more than 50% of the racks, so the 63% generalization is very misleading.
If everyone in the field hit the same number of breaks, this stat would be far more informative, even if you restrict such an analysis to players who cashed.