They work , some times the bees don't find them , try poring some of the syrup down the side of the entrance , I usually splash some down by lifting the upside down cup and scooping some syrup down the hole.
If this still doesnt work tap the glass for a while till the bees come to see whats going on and find the syrup :)
they are good for feeding without opening the hive , and no risk for robbing.
I use them to stimulate the hive to get the queen to lay more eggs even when the flow stops , build up in spring , and build up for healthy winter bees that can start the hive in the spring.
This is done by giving them small portions over a long time .
mvh edward :-P