my mother was a mostly-deaf lip-reader. She needed conversational context in order to keep up 'legibly'; and it created a lot of fun between the two of us when she would come up with an oddball question or comment that had nothing to do with the conversation once-in-awhile when her guesses failed spectacularly.
With context, though, it's a great tool. She and I used to watch crime dramas with the sound off late at night and never miss a beat. It feels if you're trying to transcribe something that has a lot of structural context the success rate is higher than 50%, but I don't know that formally.
It's still a tool I use in conversation. Even with good hearing it's tough to hear people in crowded restaurants or concert venues, lip-reading helps immensely.
