Am tempted to give one of these a try, but I guess you'd need to stock up & give it a couple of months to see if they really work (if they work for you at all!) Plus it would be mostly an experiment for my own curiosity doing more wanking than sex these days.
FWIW, I always found that saving up a big load for a punt didn't seem to work out as I planned, there'd be plenty of cum but not the sensation as I just couldn't hold off. Not enough build up I suppose. However, once that turned out to be the warm-up for one of the biggest orgasms I've ever had, so maybe having more in the tank for round two helped!