I did take an oral board course.
going right in to Pain practice solo did not lend itself to a lot of colleagues being around to do board prep with.
if you want to spend the money as part of CME, try something like Michael Ho's course. at the very least, you get contact people to Skype with and practice oral board scenarios. its a lot of $$$ though. get your job to pay for it!
you can go cheap and are just as likely to pass. buy a used copy of one of the review books to do light reading, but most importantly, practice. practice with colleagues - facetime, Skype, over the phone for the technically challenged. do them in the car by yourself. with your pet, especially if it a loud obnoxious bird, with residency friends and attendings. do them in a house, with a mouse, here and there, anywhere... make sure that those people are critical towards your answers.