Would you like me to show you your fate?
So I used the deck of many things in my dreamscape game last night - here's how it all came about, and the way I integrated it into the game without it being a mere 'treasure' item.
The PC's were walking through a chamber at the heart of the dream realm. They were striding down a long carpet that detailed the history of the campaign world in a colourful and intricate tapestry. Suddenly, one of them was sucked in to the very fabric of the carpet, turning into thread and stitch before the eyes of the other PC. To the player on the outside (who made his save) it appeared that the PC was now a static part of the tapesty. The player in the actual carpet, however, was still alive and part of a new world - one where everything (himself included) was made of intricate embroidery! (This was in the dream realm, remember) There he encountered a gypsy witch who told him of a pitched battle - between the Gypsies (the last remnant of a old, old sorcerous family) and the Demon Lord of Fate - for the souls of the PC's. Each group was tugging and twisting at the strands of fate to ensure that the certain prophecized events would fall the right way, and the moment the situation was precariously balanced.
The Gypsy offered the PC an opportunity to do a reading to help the party in it's journey, but warned him that with the demon lord watching there was as much risk that some of the cards could lead to ill as to good. The PC decided to take the risk, and asked for the Gypsy to do a seven card spread! I was using a real Tarot deck, so there was actually little chance of his turning up a really nasty card, but the risk was there. As it happens he only turned up a couple of the actual 'Deck of Many Things' cards, one being a magical item, (he got a lost item back) and the other putting him 'half way into the next level upon defeating his next foe'. He was a bit disappointed, I think - he had been wanting to use a Deck of Many Things so I gave him a couple of the 'normal' cards from the spread to use in the future to influence a situation the way of the PC's (we have rules guiding the use of magic to influence fate).
It didn't destroy the campaign, but it could have if he had pulled The Void or something, and certainly other cards could have damaged the story arc really badly, so I won't be pushing my luck by using it again in a hurry. Luckily for us this time it turned out OK. The risk is part of the fun, I guess.
EDIT: Oh, and this is our groups longest running campaign BTW. Certainly not a once of or a throw away session or anything like that. I don't know if there would be much point using something like the deck when there is nothing really at stake