I find the card broken enough that I won't play it.
Plain and simply, flip for auto-knock is broken. Yes, ok, It's a flip, I get that, and yes it works "50%" of the time.
But there is absolutely nothing "fair" about having a 200 hp creature on the board having it blown away by a coin flip.
Here's why Fainting Spell is broken: Large Creatures, like Torterra for example, are tuned by PUSA to be slow. They can do alot of damage but they take time and effort to setup. This is balanced against creatures like gengar, machamp or ramparados which require small amounts of energy for mid sized attacks with reasonable hit points.
Fainting Spell throws out the balance mechanic. Simply put, you are punnished by playing a slow deck against fainting spell, because while it takes gengar a single lucky turn to setup, you cannot setup a larger creature in that amount of time: giving a single flip the game edge.
I have seen way too many times where that single fainting spell flip is the change in the game for the gengar player...having any ability that can turn the game on it's head with the toss of a single coin or roll is wrong.
For those of you saying "Gengar is easy, just play around him", I've got to ask what kind of deck you're using, something likely fast and agile. Gengar is one of the main reasons why large tank decks that aren't SP don't work right now...because there's always that likelyhood that your 2 or 3 round setup will get blown away by a single flip of a coin.
Auto Knockout is broken and cheap. Gengar decks aren't overpowered and they're CERTAINLY not BDIF but when you find a deck like Gengar toward the top of the heap in pretty much all the areas across the US in tournaments it tells you there's something special and a bit "overpowered" about the card.
Fainting Spell is it.