I have used Mintpal, CryptoRush, Cryptsy and Circuex. They all had porblems. I hope Poloniex will be an exception.
It's weekend and they is a problem with the transaction, in the whole network. So there is no real problem. I had 2 deposits (BTC/Stellar) and not problem.
Yesterday my withdrawals also went through.
So far, Polonies is a good exchange. The volume there is also very big. I hope they can use the fees earned to boost the security.