It's still in its infancy in terms of effectiveness, but judging what Darren Berry said, the teams understand it fairly well; they know whatever the situation, a strong partnership will use it better than one or two unestablished batsmen. Just finding that right partnership seems tricky. All said, it is meant to be a gamble, take it early and maybe you will get a better time to use it later, take it late and maybe you missed the best of your chances.
Judging from how one-dayers are played, I think it should definitely not be used in the last 10 overs, as teams go quickly regardless of how many fielders are on the boundary. However, between 20 and 40 is where batsmen tend to slow down the most and it is here that quite often the 5 over run rate could double and change the game somewhat. It's a period where a part-timer can be singled out, if not, then maybe a death bowler can be expended a little.
I think Cullen bowled quite well today, got Cowan as classically as you like and got both of NSW's most dangerous players at the right times. One has to remember that the Redbacks don't have any great fielders like the guys that play ODIs and today they missed a lot of uncomplicated catches. THere are a number of players who don't pressure the batsmen out of trying to run quick singles.
Nevertheless, NSW smote SA hard for choosing to field and in spite of causing early difficulty with the greeness, this will be tough for the home side.