I've only used them once and kind of had the same experience. It was a zebra print over buttercream. The stripes went on great but the next day they had loose spots. I am interested to see what others say.
12x18 are pretty large and hard to handle if you don't usually work with them. I don't have a cooling rack that large so I use one of the extra racks from my oven when I have to make a 12x18. I much prefer working with 11x15 even if it means making an extra cake.
When I trim the cake tops I use those. I roll them up jelly roll style with icing or other filling. Wrap it in cling wrap and freeze, the slice while semi frozen. It's still cake scraps, but nicer looking.