When I submitted my eAPR I did not know any cases of rejections after RPRF, that's why I did not pay it upfront. I thought it might me very beneficial to get this request in the middle of the process and stay relaxed. However in January several cases of refusals after RPRF followed and now it seems that one cannot feel totally secure untill getting this PPR letter.
As far as processing time is concerned it's quite different. Lots of applications are pending several months after RPRF, others get PPR in few days. You never know which group applies to you: lucky men or long waiters.
What I know for sure is that since RPRF every single incoming email makes me shiver ;D