CSRS folks indeed had a 2.5% multiplier and zero vesting fee, those days are long gone. FERS folks are a piddly 1%, and the vesting fee for folks hired after 2014 is a whopping 4.4%. That's a huge delta. A CSRS complaining about his/her pension being peanuts really needs to look in the mirror, especially compared to FERS people of post 2014.
For comparison, an Active Duty military retirement is a 2.5% multiplier, but most folks can't attain more than 20 years. Most don't get an active duty retirement AT ALL. The replacement rates hardly ever exceed 50% as a result. After you account for the portions of military income that are not counted in the retirement pay (only basic pay is counted, which is 75% of gross for a mil guy, the rest is tax free allowances that don't count) you end up with a 60% take-home paycut depending on your state income tax specifics. BL it's not enough to retire on for most of us (aka second career city). New military entrants as of 2018 are now under the BRS system, which lowers that multiplier to 2, making it even less attractive to stay in indentured servitude for 20 years.
Suffice to say I would actually pinch my nose and go airlines and their B-fund before dabbling in the FERS retirement (Air Reserve Technicians fall under FERS) if an Active Duty retirement wasn't attainable by yours truly. The LEO portion of FERS has higher multiplier and some sweeter rules for withdrawal, as has been highlighted by others in the thread already. And I second those comments as well; it may just be the only FERS variant worth pursuing over other state retirement options if I was absolutely forced into the choice.
For clarification, only military members who do not currently receive retirement pay are eligible to buy their active duty years for credit into a FERS retirement. That is a huge opportunity cost because recall that an Active Duty year is a 2.5% multiplier, whereas a creditable FERS year pays a multiplier of only 1%. If you are not staying in the military anyways (for QOL reasons, and Lord knows there's plenty of reasons to quit these days) then sure, the opportunity cost of taking such an otherwise huge retirement multiplier paycut is zero. So it's hardly the benefit being touted. Furthermore, the true double dipping can only be incurred by Reservists who draw a Reserve retirement pay, where those Active Duty years can be used concurrently for Reserve retirement pay, AND creditable years towards FERS retirement. Otherwise, no such double dipping, especially for those drawing an active duty pension. Receiving two pensions with non-overlapping creditable years is NOT double dipping. Receiving a federal pension and using bought back active duty years as part of the creditable year computation is also NOT double dipping, as you're not receiving a military pension from those years as part of the condition of being allowed to buy back into FERS.