I normally watch English-language video with subtitles on. Habit, I suppose. I find they help more often than they hinder, especially if you're playing at accelerated speed (a typical post-1990s documentary is bearably paced at 1.2-1.5×), or someone's gone over the top with the background music.
ETA: Since I use headphones for computer audio, I tend to watch short internet videos propagated by news sites and social media without sound. If they don't have subs (or make sense without audio), I usually can't be bothered.
Quality varies enormously, especially if you've gone fishing for pirate subtitles files in the seedier parts of the internet. Not just the distinction between verbatim transcription (translators, for the use of - it's usually a bit too much to read), English for foreign language users and English for the hearing impaired. A good hearing impaired subtitle track is a work of art[1].
[1] I recall introducing a Deaf friend to Ferris Bueller's Day Off and cringing as every non-visual gag fell flat on its face. The subtitles had simplified for language and verbosity, and managed to whittle away most of the humour and comic timing.