Took out the added years last year. The hardest part was trying to work out if it would be worth it as the Pay and Pensions people were next to useless in terms of providing sufficient information to enable me to make an informed decision. Moral of the story - FPS are probably the people to speak to.
That said, I joined close to 23 bday; although I was 22 at the time, they base calculations on how many years you can buy from your next birthday, so I was allowed to buy just under 2 years extra service. As a Flt Lt, that costs me abotu £100/month. But it is buying into a pretty safe (for now!)investment and as it comes out at source, you don't miss it after a couple of months.
If you can do it, and don't need to consider spouse / family, then it is probably a good thing to consider. But get professional advice based on your personal circumstances rather than relying on PPrune advice!