I exclusive use coinbase because its local and insurer backed. But I have recently starting using Poloniex solely because it has access to more altcoins. While i have never had any problems besides the occasional lag when loading a page or new coin, i have read and heard personal accounts from many regarding countless issues and minimum support. I would not advise keeping a large amount of coin in polo, too many issues can arise and the issue becomes compounded when involving margin trading and coin loans. Be smart with your money always and don't rely on polo bailing you out, they wont!