Launched in 2014 by Tristan D'Agostar in Wilmington USA Poloniex exchange is one of the oldest exchange in the crypto market. Though based in USA it has seized to be available to citizens of US due to certain laws.
In its early days poloniex was hacked but since then it has restrategized its security system and since 2014 it has not experience hacking again. It store majority of customers deposit in a cold storage offline. Even the users whom it's coin was stolen then have been paid back by the exchange. To help its users there are 3 levels of verifications depending on your withdrawal needs.
You are free to deposit coins that are been supported by the exchange with no cost. When withdrawing your coins you only need to pay the blockchain network fee alone. When trading you will pay some fee as taker or maker. If you are maker you pay 0.15 percent, but if a taker you pay 0.25 percent but with the increase on your volume of trading your fees will be reducing.
Poloniex has trading tools for both experienced and new traders. The customer support service need to be improved upon, because it use to take too much time before they will answer you if you contact them by email.
Conclusively, if you are interested in using poloniex there are lot of features that you will enjoy using it despite its inefficiency.