i have not heard about coin-mu. so i dont recommend you to exchange a huge sum there. but i think cex.io, not to confuse between cex.io and c-cex.com, is legit and it is also available for US residents. they also take less fees, 0.2% for btc/usd exchange. but note that they will be under maintainence today at 9pm forum time so you will not be able to exchange around this time period. i got this information from their twitter post:
https://www.twitter.com/cex_io/status/753878071460192256