Had them for years. They are best for pockets 4.5 inches +, but could be used all the way down to 4.25 inches. At that size you have NO margin for error to speak of along the rail. If your pocket is smaller than that, I doubt rail shots are even realistically makeable. I recommend glueing pool cloth to these. It makes them behave slightly more natural if you intend to play with them on. Usually, though, I play with the ones without cloth (have two sets)
Sometimes the spring gets in the way of a shot. I then usually remove the offending aid, and put it back after. Also, some pocket cuts will force the reducer to eject themselves out of the pocket at random times. It's not that common but it does happen now and then.
I'd have to say that, after spending time with these, chinese 8 ball tables were not a big leap at all. I play all the tight pocket games, snooker, chinese 8 ball, blackball and I have to say, these pocket reducers on 4.25 inch pockets are absolutely brutally difficult. My high run on these in straight pool is in the 30's only, and not for lack of trying. I think I maybe could improve on this if I used the ones with cloth on more. Good luck.