as backend we had oracle. at first we tried oracle/linux (just released). but we never managed make it work (oracle engineers that came to us failed as well). So we got dedicated sun server for it.
One day I was bored, installed mysql on my workstation, made a changes in couple of queries and all of sudden i got x20 performance of sun box with oracle. Lead developer said that it's bad solution as mysql doesn't properly supports referential integrity (we didn't actually used it in oracle iirc)