Wow, feels like an eternity, but my X5 has finally arrived. I know many, many others have received their units and posted their impressions so I'll keep mine brief.
For starters, though I expected it I really can't say how much I appreciate that FiiO included a silicon cover for the player. It's nice not to worry about small dings and scuffs right out of the box. I do wish it allowed access to the TF card slots and charge light indicator, but still, a very nice inclusion by FiiO.
I briefly played with the unit with the included fw 1.2 before upgrading to 1.23beta. I can confirm that 1.23 solves to a large extent most of the UI lag issues, though I found them minor (still a little annoying) on 1.2, everything is snappy and instant on 1.23beta excepting there is still some lag when adjusting volume (i.e. holding the volume adjuster doesn't immediately stop when you let go, etc). But it's very good, I don't anticipate it being bothersome.
I have experienced the well noted minor bug of the first couple seconds of songs being cut off when I begin playing. However, skipping and restarting songs is fast and flawless. I find the sound neutral. As has been stated, the sound quality is excellent. Not much I can add to what others have said there.
Minor nag, I would like to see the option to have the screen timeout in less than 30seconds. I tend to fiddle/skip/change songs with my players while listening too often and like the screen to not stay on for too long. I know, I should just stop fiddling.
Looking forward to listening for many hours each day for the foreseeable future.