Marty B, just to give you a recent experience with the OPM backlog and getting finalized. I retired as a FERS guy on 6/30/12, got assigned an OPM Specialist on 10/24/12 and was finalized on 11/29/12. First full annuity paycheck will be 1/2/13.
Interim pay was about 2/3 the final number. "Adjustment Pay" to recoup the pay you didn't get while in Interim status was deposited within less than a week of getting finalized.
The OPM Specialist was easy to work with and I'd have been finalized sooner, but had to provide additional "proof" for some Reserve active duty days served prior to becoming an ART that I bought back but weren't covered by a DD214.
Your mileage may vary, but...