My experience of using the HH Diamond room availability guarantee has been, to say the least painful. If the hotel is full, then even when in the Hilton system as a Diamond HH member, no availability is shown, and when I've contacted the Diamond Desk or the hotel itself, they've thrown all manner of excuses at me. I can't actuallt remember one instance where using the guarantee has got me a room.

Imagine my surprise tonight when I made a Marriott reservation through their website (I'm MR Platinum as well as HH Diamond, and they have a similar guarantee).

The reservation processed and when looking at the confirmation page, there was a small note to say the hotel was full, but they'd used their Platinum Guarantee to give me a room. Fantastic system and one that Hilton could do with emulating.