Don't use them myself, but barley is quite a good, cost-effective source of calories for a poor doer. Barley rings are cooked, which makes the feed more digestible, plus a source of oil is included, which increases the calories and helps put weight on.
Disadvantages are that cereals do not agree with all horses - makes some fizzy and not a good feed for laminitics.
Tend to feed them in the winter to help maintain condition, soak them in hot water for 10 mins before adding to rest of feed, smells lovely and horses seem to appreciate it
i fed them to my old gelding. However, they didn't make much difference weight wise but he was 40 years of age. But he loved them and I fed them cos he loved them as it was difficult at this age to find stuff he would be able to eat and chew.
I used them on my youngster last winter with amazing results. She was turned away and dropped off far too much. I put her on barley rings. Alfa A oil and a balancer and she looked fab in just 3 weeks.
i use them for my TB ex racer and he does realy well on them. much more cost effective than a lot of conditioning cubes too, and the added linseed gives a lovely coat