I’ve run the gamut with barbell declines. A long time ago, I used declines in every chest workout. More recently, I just throw them in as the first (main) chest exercise for about a month every third or fourth month.
I have seen chest gains from declines, and I think there are many (but not all) people who can benefit from them. They’re typically hit or miss. For me, I just hit a natural groove on declines and can pile on the weight without shoulder trouble, whereas some people’s shoulders won’t allow them to do much. I think that if you can perform them without pain, they’re an excellent supplementary exercise, and as far as pectoral hypertrophy goes, can almost replace flat benches for some people.
The only reason I don’t use declines regularly is that if I substitute them for flat benches, my flat goes way down after a few weeks of declines, so there’s very little strength transfer from declines to flat. 
Thus, I only use declines when trying to build up my man-titties. 
It’s also fun moving a lot more weight than on flat bench, though. When I was declining regularly, I could decline 375 but only flat bench 315. Now they’ve almost met in the middle.