OK, here is another "I just got my x5" post. I have not played with the UI enough to form an opinion. My very early impression is that I can live with it just fine.
Overall, I really like the x5. The sound is very clean and detailed. It is however, even better when played through my iFi Nano iCan amp. Adding the amp gives significantly stronger but still very clean bass. I don't think the bass is over boosted with this pairing (EQ flat on x5, no bass boost on iCan). Rather, I think it is appropriate and sounds very good. My comparison is a calibrated home theater set up with a subwoofer.
One bit of advice, as others have said make sure you format your memory card before you transfer music to it. I wasted an hour+ transferring music without formatting my SD card only to discover that the x5 couldn't read it. I then formatted the card in the x5, went through the copying again and now all is well.
Here is another tip for folks with big music libraries. Get a fast SD card (at least 20 MB/sec write speed) and a USB3 card reader for your computer. Transfers will be 5x to 10x faster than going through the x5 or using a USB2 card reader.