The backward compatibility program for Xbox One was announced during E3 2015 and released later that year. Since then, a whopping 840 million hours has been spent on games from previous generations for the Xbox One according to Microsoft.
Since we don't have any official sales figures for Xbox One, the best number we have comes from the insider and Niko Partner analyst Daniel "ZhygeEx" Ahmad, and he claims roughly 35 million Xbox One has been sold. That means that the average Xbox One gamer has been playing backwards compatible games for about 24 hours, we assume some have played a lot more while others have barely used it.
This number was be seen as quite impressive, and the feature has turned into a real draw for Microsoft and the Xbox One. Combined that with the 4K support on many older titles for Xbox One X that really make them look better than many full priced remasters released this generation, and hopefully, the success means the support will continue to expand.