If yes, what do you think about them?
I used to. My primary trading was done at 10x futures. However, with all the rumors surrounding Okcoin (bad security protocols, front running their customers, making the market with un-backed accounts) I moved my funds elsewhere.
Sticking to BTC-E, Bitfinex, and Coinbase. But I do miss the low/no fees.
You can try HUOBI to lternative market, if you comfortable with Okcoin, Huobi will be comfortable too... and you must remember we can trade without fee on HUOBI and low interest Loan