I have never used them, outside of a few gc ones to get my mount.

I think i have all of them available atm, i was thinking of using them to lvl gld and acn up to 15 but I keep reading I should save them.

If i use them to lvl those two classes, and go from say 99 down to 50, do I start building them back up every 3 hours again? Will it eventually go back up to 99?

Also do they payout more gil then normal quests?