So far I have used RKT twice... once for the new Jack Skellington head and once for a project that failed big time.
For the Jack head I made just melted marshmallows and RK.. no butter. Compacted them in bowls and let them set... they hardened up nicely. Covered in BC and fondant. The fondant wouldn't have stuck without BC... they dry hard and it's pretty smooth as in un-sticky.
For my failed project, I have no idea why but I made the RKT the same and they never completely hardened... even after 48 hours. One specific piece I needed to conform to a shape to look right... it was still flexible and fell apart. I covered some other pieces I made in BC, this time the fondant fell off without it being on (I had several small pieces I could cover so I tried it).
I'll definately use them again... I just wish I knew a sure fire way to make them harden. I guess you could make it a week before and let it dry.. but then what is the sense of saying the cake is edible if the RKT are like rocks? I'm going to try cooking the MM longer next time like suggested. I think I did that with Jack.
Anyway, that is my experience. I'm kind of a "jump in there and try it" type of guy, so I say GO FOR IT!!