Personally I have had nothing but less than good results with MP4Nation.
One order was for an iBasso product which they stated was in stock. After two weeks and no replies to my "tickets," they offered a refund through PayPal. It then took them over a week to make the refund transaction. They never were able to give a satisfactory answer to why it was specified as "in stock" and obviously wasn't.
Seems their statement that most orders are shipped within 24-48 hours is misleading. I've had two previous orders with shipping information supposedly sent to FedEx but tracking information wasn't available for five days after MP4Nation stated the items shipped. I believe they make out shipping labels but don't really ship until some days later. Eventually I received the merchandise but only after a couple weeks and this was using their FedEx Express option. One item was DOA but I decided to eat the cost rather than going through the hassle of a return with them.
Prices are good, but their customer service still sucks. Answers to questions are ambiguous, and getting a straight answer is something elusive and time-consuming.
I may be one of the unlucky ones and your transactions with them may very well be positive.
I ordered the DX50 from them - it shipped a week later than what they had indicated on their website.
I ordered and then cancelled an X5. They said they'd process the refund in a few days, but forgot. I sent them a reminder and it got done in a few hours.
They are a little prone to missing deadlines on newly-released stuff and obviously arent the best-organized - but IMO, for pretty much everyone outside the US (ie, where instant gratification isnt the norm), it isnt such a big deal (they arent out to rip you off or anything). They have great prices and offer free international Fedex shipping - that's huge. I am ok with sending them a reminder if need be.
And that's not mp4nation's fault. They are dependent on what suppliers tell them. Anyone who has followed the FiiO development threads for new releases of their products know that there are often delays from the manufacturer with a new product release. Or even delays with a product that has gone out of stock and the manufacturer is doing another product run. All they can do is give the projected date of availability. And while I have not ordered from them, it's my understanding that they generally do a pretty good job of getting the new products out there once they receive them, even if there are some delays.