From the Choate report

The USPTO has recieved 9060 requests for ex parte reexaminations between the start of the process in July 1981 and December 31, 2007. Of these;

"Conclusion"

" Ex Parte proceedings at the USPTO are an inexpensive, expedited way to determine patent validity without litigation. Since the availabilityof this process in 1981, it has been used 9000 times both to validate and strengthen patent claims by owners and to challenge them by third parties and the USPTO."

"The ex parte reexamination reveals that third party challenges resulted in cancellation of all claims in only 10 percent of the cases. Of the 6066 ex parte reexamination certificates issued between 1981 and December 31, 2007, 3874 certificates had one or more claim changed (64.0%). And 26 percent had all claims confirmed. This ex parte reexamination data strongly suggests that the overwhelming majority of patents being issued by the USPTO are able to withstand challenge and reexamination."