Bittrex is probably one of the best out there. I have good experiences with Cryptopia and C-CEX too btw, although they are just smaller exchanges. And I am ashamed to say I like Yobit too, haha.
I only see complaints about bittrex on this forum. Are you selling bitcoin and withdrawing USD or EUR with bittrex?
Bittrex doesn't have any fiat gateways, you can only trade between cryptos not directly with any real world currency. I don't know where you see complaints though its the smoothest exchange out there and never had any lag issues.