I'm watching YouTube videos with subtitles for my wife, who doesn't speak English. For videos on basic topics where people speak clear, unaccented English, they work fine (i.e. you usually get what people are saying). If the topic is in any way unusual, the recording quality is poor, or people have accents, the results very quickly turn into a garbled mess that is incomprehensible at best, and misleading (i.e. the subtitles seem coherent, but are wrong) at worst.