I'd say about 100% success rate with fully uninstalling unintentional /unwanted downloaded apps etc,
More like 30- 50% with seriously messed up systems. Not unusual it either fails or makes things worse
Always worth trying it but most of my faith is in a combination of full system disc images of the 'clean new system' (just install windows, maybe office, licence regirtration if required etc and do all updates) and recent backups of email and other user files, photos music data etc.