My first experience as a Diamond resulted in my not getting the room type I reserved (even though it was confirmed by the Diamond desk), and actually being given a smoking room when my preferences are non smoking (at least I got a non smoking after much discussion with the front desk). Bright side is that they are refunding my money after much complaining to the Diamond desk. So my guess is that not only are you not guaranteed a room at a sold out hotel via Diamond force, you may not even get the room type confirmed! I guess Diamond is not all that I expected, but hard to say after just 1 HORRIBLE experience.