I had my Flak Jackets re-glazed by my local opticians (who sent them off to some wizard they use), they did a fantastic job and were cheaper than the online quotes. I was told that whoever the they use could do nearly any sports glasses with a better quality lens than the original Oakley prescriptions I previously used. Varifocals for me have been a nightmare as regards getting the "vari level" where I like it, so it was reassuring to use the optician I normally go to (sorry don't know the technical terms for lenses).
Having said that, as yoav says, the frames are the cheap part, mine went brittle and cracked, but as original Flak Jackets fitted well and are still available I stuck with them.