Paxful -- don't trust anyone on this exchange. It doesn't matter if they have 800,000 positive feedback or 0 feedback, sooner or later someone will screw you. I have multiple stories of scammers, whether it be paypal or gifts cards or anything, really, there is a scam with all payment methods. Better to simply stay away from it completely.
Poloniex -- suddenly holding everyone's funds and locking accounts? SCAM.
Cryptopia -- constant problems of coins being held, deposits not showing up, withdrawals not going through, and even after all the hype about the new CEO, they STILL have these problems. STAY AWAY.
Yobit -- many, many complaints of scams here.
What are your stories? Please share them here.