This card is a wonderful, wonderful card, which if played right, can swing the balance of power into your favor easily. From keeping your win-condition critters alive to stealing other players critters, and for being a card that's really hard for your opponent to get rid of, I love this card.
On the surface of it, Fool's Demise is a very simple card. Enchant a creature, when that creature dies, it comes back into play under your control. Where it gets interesting is that Fool's Demise is one of 5 cards released in Time Spiral that, when put into graveyard from play, bounce back to your hand.
What does this mean for you? It means it's reusable. Put it on a creature you control. If that creature dies, the creature comes back into play, and Fool's Demise comes back to your hand. Play Fool's Demise again, and you have a creature that pretty much can't die.
Strategies and game play:
There are two main uses for this card. The first, and most obvious, like I stated above, are to keep your critters on the board. The second, and much more evil way of using it, is to steal your opponents creatures. Play it on one of their creatures, and when it dies, it's comes back into play as your creature. I'll get into some more specific combos in the next section.
Combos with other cards:
Sacrifice this creature - play it on a creature of yours that has a sacrifice cost. You sacrifice the creature, gain whatever bonus it gives you, but the creature comes right back out into play, and you get Fool's Demise back, to play it again.
Burn Spells - It's a very easy (not to mention very evil) way to steal your opponent's creatures. Cast it on someone else's creature, kill that creature with direct damage, and now it's your creature. It's especially fun to do when the creature in question has great advantages for the original owner, but when for you it's just a chump blocker. Example: I played it on my opponent's Urborg Syphon Mage. For him, it's a great card, because it enabled him to use madness costs, as well as to gain life. For me, since I wasn't running black mana, I couldn't use Urborg's abilites. However, I not only stopped him from slurping away my life, but I gained a chump blocker for when he attacked me. Another player then pointed at me and said "EVIL!" I had to agree.
Thwippers - This is the combo I use with it, and it's simliar to burn spells, but instead of using another spell to do damage, you use creatures that tap to deal damage to target creature or player. The downside to this is that it leaves you vunerable, as you have to use several thwippers, which can leave you open to a more traditional attack.
Fading - although not a mechanic encountered much anymore, creatures with fading are only in play for a limited amount of turns, then they went into your graveyard. With Fool's Demise, however, they come back, AND their fading counter resets, giving you several more turns to use that creature.
World Destruction - there are several cards in the game that basically say "beep this, I'm killing everything." Fool's Demise would save your creature from that fate. You could play Fool's Demise on your own creature, and then nuke the world, which would kill everything else but save your own creature. You could use it to save your creature against someone else's "nuke the world" spell. You could play it on their giant sized creature, and then nuke the world, meaning you'd have their giant creature and no one else would have anything.
I'm sure there are several other interesting combos with Fool's Demise, but those are some of the biggest ones.
Ways to counteract it:
Fool's Demise is actually suprisingly hard to counteract, once it's on the table. Because of it's wording, however, if you are forced to discard it from your hand or from your library, it goes into the graveyard and stays there. Also, effects that would remove it from the game are also effective, as it only returns to your hand if it's placed in the graveyard. Since most players only run standard disenchants, however, oftentimes you'll find it merely going back to your hand.
It's reusable. Bring a creature back into play, get it back into your hand to play again. Wait a turn, untap, upkeep, draw, play it again.
It's fairly expensive. 4U to play, and it comes back to your hand, instead of staying attached to the creature. If using it to capture an opponent's creature, you still have to kill that creature, which usually requires more mana expenditure in the form of burn spells, or you have to attack, and hope they choose to use that creature as a blocker. Also, it's an enchant, which means you can only play it during your main phases - you can't use it to react to something.
I'd give this card a 6. It's got some great possibilities, but it's expensive to use, and using it against your opponent's creatures makes you an instant target. Also, while it's possible to win the game with this as your lynchpin, it's not likely to happen, as you rarely have the mana to use it more than once a turn at most, and that's assuming you have a way to kill their creature.