I just got my X5 in Australia, and doing some initial comparisons with my good old X3.
Sound wise, I found X3 and X5 is very different. X3 is a bit more lively and exciting, whereas X5 sounds cleaner but has far better 3D sound stage and imaging. It's almost like X3 makes any recordings sound energetic and in-your-face, whereas X5 makes bad recordings sound bad. An analogy may be X3 is like Grado and X5 is like Beyerdynamic, if you know what I mean.
One thing I found a bit disappointing is that X5 has a nice amp section but it still struggles with high impedance headphones like 3~600 ohm. Power output is only as good as USB bus powered headphone amps, not desktop amps with a huge power supply. It does make the headphones loud enough, but it becomes sloppy and thin when using high impedance headphones.
The interface is also not as good as I hoped. It's a glorified high-res version of X3. I wish it was better. I am annoyed that, as far as I know, I still can't choose Genre>Artist>Album, etc. If I choose a Genre, it simply plays every song in that Genre in alphabetical order, which is ridiculous and useless.
Not sure if it was reported earlier, but I once had one weird experience where the LCD screen was flipped like a mirror image. I restarted the X5 and it went back to normal again.