Bitfinex is the biggest one and the most reliable. Sometimes it`s a little bit to slow, but I see their improvements everyday. New library, new tools, application. I think finally they come to a perfect quality.
I cant say I ever had any issues with it, if so it was so insignificant I cant even remember exactly. The only con I see in it that its tough for beginners and by ‘tough’ I mean difficult to use UI first of all. For experienced traders its like paradise. Looking forward for the updated Bitfinex coming in a few days. Its gonna be more secure and faster.