I can only speak for my own experiences, but I have no complaints whatsoever. Their customer support (by email) was polite, clear and expedient. My package was delivered from them to Sweden in just 5 or 4 working days in mint condition and at the same price as their shipping cost calculator suggested.
I have read that they donät have any phone support, which I have no experiences of since I never tried to call them.
It may also be that other companies offer better pricing on the services but I don't consider 65 dollars (+ 10 dollars initial sign-up fee) bad for a pair of shoes (4.3lbs) from Florida to Sweden.
I was completely overwhelmed by their great email customer service, of a quality that I have never experiences before. This might have a great influence on my impression of MyUS, but as I wrote above, I had no negative experiences from the delivery service itself either.
Karl (Sweden) (Shipped a pair of shoes in the beginning of April 2012)