I love pop-corn (who doesn't I guess), so I think it might be used in beer during mash.
Has anyone used them? How did it work? How should I do it? Should I expect something interesting from it?
Yes you can mash with popped corn, it can be thought of basically as torrified corn.
You do not, however want to get the buttery flavor that we typically associate with popcorn in your beer so make sure you use an air popper beforehand.
You are going to get the typical corn flavors come through, not necessarily a "popcorn" flavor, if that is what you are shooting for (which you shouldn't be).
As far as how you should do it. I would pop it first and then mash it down so that you can get most of the air out. I can only imagine trying to fit a couple pounds of popped corn into a mash...
Use it how you would typically use corn in your mash.
I have made two batches of beer using air popped corn. They were extract ales, my first batch used 1lb of corn, my second batch used 2 lbs of popped corn, it takes a while to dissolve the popped corn in a kettle, but it will fit. The first time I had it was from a micro brewery for a popcorn festival, and I was surprised as it was good, so I just had make some, overall it wasn't bad, one of my workmates was sure I had added salt, but that is the power of suggestion, it being popcorn and all, now I learned not to say anything until they have had a bottle or so, it's interesting how the imagination works on people. PS airpopping(No oil) negates mashing as the starches are modified for direct conversion with your malts. Any oisl will screw everything up.