I trust Poloniex already for a while and I can say with my heart that I fully trust them and that I can recommend them to you guys because there is not wrong with their platform and services.
There is already news that Poloniex is already having issues in their security that needs fixing. By all means use them to trade cryptocoins but do not store them there for a long time. Even if they are trustworthy they are only human. They are prone to make mistakes and there are smart hackers out there always trying to hack in their system. Blockchain.info was hacked a few days ago. That is a bitcoin site that takes security very seriously but they still got hacked.