Excuciate does help a lot, but it will not guarantee the crit. I would say just by eyeballing it, it's anywhere from 15-30% fail rate on the spell you want it to crit. on. I tried running Thundaga into my rotation for Ifrit, but the second I failed to crit Thundaga I instantly fell behind in DPS and MP management. It's a gamble. You need it to crit to make it worthwhile (in 1.20), should change with Convert. Convert should allow more room for error.
