Though a good kill, the action would have been far more significant back in the days when Rustock was in heavy use.
With the demise of Spamit, Rustock's usage dropped off last year as well, and although it is rated as one of the largest botnets in terms of numbers, it has been rather a low volume spam machine lately. In August last year, 60% of bot-spewed spam was Rustock, but it took a steep dive in late September. In the past week it was ranked at #8 in the top 13 by M86 in their graph of "Spambot Activity Over Time" - http://www.m86security.com/labs/spam_statistics.asp
(it's the pink line).
Predictions of significant drops in global spamming as a result of this operation are ill founded.
Predictions of an immediate drop in spam from Rustock are totally credible.