I have been playing with my X5 for a few days now. I really love the sound and build quality, although like others have said, the wheel isn't the best.
I do have a few gripes though. I think the thing that annoys me the most is navigating through artists. I have hundreds of artists to scroll through, so I find it a real pain that there isn't some sort of fast scroll. I think this is something Fiio really need to sort out as I have seen others complain about this also.
I have it almost setup the way I like, in a way which may seem odd to some people. I have all my music spanned over two 128g cards. I also have portable version of MusicBee on there. So when working on my computer I launch MusicBee off the X5 and have the ability to listen to all my music. I used to carry around a portable HD, but this is so much more convenient and it's awesome to have my whole collection on the go now.
One other gripe I have is about the speed. I have setup MusicBee to export the dynamic playlists I create directly to the device. So for example I have a playlist with all my 5 star tracks. However when loading the playlist it takes an age to finish loading. Not a massive issue but just a bit of pain.
I've only had the device since Friday, so am still getting used to it, but am liking it, minus some of the gripes mentioned above.