Worst Fears was playable in Standard. Is it good in other formats like EDH?
Still, Worst Fears can be a pretty scary card. Especially late in the game, being able to control an opponent's turn can be devastating. Also, having only a single black mana in its mana cost makes it playable in a wide-array of control shells. Many players consider this card a waste of a mythic rare slot. Then again, I can understand them not wanting to print an effect like this at rare - even if it was on Mindslaver back in the original Mirrodin days. When Mindslaver was reprinted in Scars of Mirrodin it was bumped up to mythic rare.
Modern Tron makes heavy use of Mindslaver to great effect, so it was thought Worst Fears could be played a bit in Constructed. Of course, eight mana is a lot. But it did in fact sneak into a few Standard decks back in 2014. One 8th place Standard PPTQ UB Control deck ran a copy in the sideboard. Pro Ali Aintrazi even played a mainboard copy in his January 2015 Sultai Delve deck at a Standard Open in Washington DC. A "Mono Green" deck even ran one copy in its sideboard, as this Green deck splashed Black mana for Pharika, God of Affliction and Doomwake Giant. Since early 2015, it hasn't see much in the way of competitive play, in Modern, or otherwise.
Worst Fears does see some play in EDH / Commander. The Commander who's made the best use of this sorcery is Jeleva, Nephalia's Scourge. Jeleva has an ability to exile cards from the top of your deck, and you can cast any instants or sorceries from those exiled cards without paying their mana cost. This means you can play Worst Fears for free whenever you want. In other Commander decks, it's just not really worth an 8 drop slot.
Pricewise, Worst Fears sells for about $1 as of January 2017. Its foil price is about $4, which is relatively low for a mythic rare. So it's a pretty cheap addition to any Magic collection. The later in the game this card is played the better, so it's a good effect. But since you only get one shot at it, you had better play it at the perfect time. Really, it's best if you can find a way to cast it for free. Otherwise, it's not worth casting in most decks. While it had its uses back in its Standard legal days, nowadays you'd be better off with Mindslaver.