Well, I microwave them for about 20 seconds and then bend them with my hand and I was able to recover seven of the unreadable (or partially unreadable) discs, and I always kept a "master list" of what was on each CD so I could find the songs I wanted - so if I really had to I could borrow, re-rip, or download the tracks/albums I needed.
I also, as I said, would often re-rip things, so I sometimes have a better version of an album on another disc.
It's not a tragedy, but it's one of those annoyances, when I consider that I have a LOT of burned CDs with archived data that may or may not be available the next time I need one of them.
The upshot is that for the past two years I've been too lazy to make disc labels for anything - so the ones that just have a title on them in Sharpie marker all seem to be fine.
I had another set of discs that I stored horizontally, on a spindle, and those discs seem fine - I guess the pressure of the discs kept them from going concave like the ones that were stored in jewel cases did.
Anyway, I finally got 99% copied over and it's really really sweet to have access to all of this data at once.
And I still have over 100Gb free.