The only case I've noticed where it gets it wrong is when you have a quote (either single or double) immediately following a dash (endash or emdash), or after a hyphen. But those cases tend to be rare. Note: be sure "Use smart quotes" is checked in Preferences or this won't work.
[robg adds: I often have to go the other way, converting curly quotes into straight quotes. If you disable the smart quotes option in Preferences, you can then just reverse these directions -- copy and paste a smart single- or double-quote into the Find box, and then just type a single- or double-quote in the Replace box. You only have to copy one of the double-quotes; even though they look different, all will be replaced when you choose Replace All.]

