We purchased a king size adjustable bed which is 2 twin XL put together.  These sheets fit great and I have no problem with them pulling off the corners when the beds are in motion.  

2 complaints...1 is that they are not really soft but I am hoping they get that way with continued washing.  2 is that the hems are very sloppily sewn.  Eventually I will have to go in and make new hems myself but for now they are ok.