I really hope you won't be disappointed. I have to admit I didn't read this whole thread, but I skimmed through it and didn't find any negative things. I just got my X5 II last Friday. Sound quality wise I have nothing to complain about, it does sound great for the money. Build quality and software ... well, that's a different story, at least from my point of view. My unit came with the left SD card slot soldered a little off, so the left corner of the card sticks out just a little bit. I'm thinking to use some sandpaper to remove a little bit of plastic from the micro SD card, to make it fit better, but again, for a 400 euros device I wasn't expecting that. Also, the metal around the control wheel came with some small dents, nothing I can't live with but pretty disappointing nonetheless. Maybe it was just bad luck, because I didn't hear about such issues from anyone else.
I did upgrade to the latest firmware the same day I got the device, still I experienced the following issues: at first, suddenly, the unit stopped playing any tracks. It was looking that it was about to start playing, for any of the tracks I tried, but nothing else. I wanted to turn it off and then on again, but it got stuck at "SEE YOU" for about 15 minutes. I eventually had to keep the power button pressed for 6+ seconds to do a "reset". Once it restarted I realised I lost all the music on the left card; by connecting it to the computer it looked like the card was corrupted. I tried to format it in the device (System settings -> Format) but it got stuck there for tens of minutes, so I had to "reset" the device once more. Switched the cards (from one slot to another) and tried to format both again. Left card format completed in a split of a second and then I realised it wasn't actually formatted, music was still there. The card in the right slot took longer to format and eventually got formatted - all ok. I only experienced all of the above once, but I guess everyone's getting my disappointment, to have such issues the first day I got a new highly rated device. Another issue that's still going on is the battery indicator not working correctly. It shows about half charged and then shows the "battery low" message and shuts down. Happened twice to me already. Coulomb meter for precise battery level readouts being a selling point, that's another disappointment right there.
To be honest, I would probably return it if I could; I am amazed that I didn't read any complaints before, I can only assume it was just bad luck for me and I'm wondering if maybe I got a faulty unit. I'd really like to know if anyone else has similar issues...
I hope you won't have any problems and hope you enjoy your purchase. Sorry for the mostly negative tone, just wanted to share my experience and maybe get some feedback... Cheers!