I know this has pretty much been answered, but I work with several retired vets who have all bought their time back. Like everyone else here said though, you have to do the math.
All of the guys I work with retired at E-7 (except for 1, who retired at E-6). However, they are all now GS-15 level guys (I say GS-15 level, they were NSPS YA-3's and now AQDEMO NH-4's). When they did the math, the extra 20-25 years (20-25%) added to their GS-15 retirement ended up being way more a month than, so they went with it.
As others have said, you keep getting your military retirement until you "re-retire" from civil service. Also, as others have said, you keep getting your disability check from the VA forever, whether you buy back your time or not.
Last, the point that it moves you up the RIF ladder is absolutely true, and is worth considering in the current hiring freeze, RIF, buyout, etc environment.
In general, if you didn't retire from the military, buying your time back is a no-brainer. But, if you did, you pretty much have to be a GS-15 for it to be worth it, and even then only if you retired at the E-7 level. I've heard E-8 is the break-even point, so if you were E-8 or E-9, or pretty much any officer rank, it's is not going to make you any more money.