I bought one, it's Dutch (I've just woken up, can't think of the name, but I have posted before about it) and it was quite (very) expensive, but it was worth every penny.  I'd like another for my little girls (bantams) but I'm saving up, because I'd really like 2 in their run instead of just one.
What's so great about it?  I open that thing up when I'm cleaning up and there's food in there.  Lots of food.  I don't know where it was all going before (all kicked into the dirt, I expect, or eaten by other critters who now can't get to it) but the amount of feed the big girls get through has been substantially reduced (I never really paid enough attention to say for sure, but I suspect that I've probably recouped the cost of it in having had to buy fewer bags of crumble)
It was easy to show them what to do.  My big girls are quite greedy, the motivation to use it was quite high.  
 
EDIT: mine is big, it holds like 20kg or something, so I don't have to refill it often.
FURTHER EDIT:  
https://www.olba.com/en/feedomatic-automatic-feeder-20kg
Here it is.  I have this thing out in the run, the crumble stays dry inside, the rain doesn't get in.  It doesn't clog up or anything.  All I do is open the lid and add crumble when needed.