I find the UI, when not buggy, to function really well. Rather smooth at times, and to be honest, those who are complaining are those who are taking things for granted. Its not as bad as it is said, and it surely functions on par with some other DAPs. I guess all of this would be useless information, since I came from a Colorfly C3, which was jam packed with bugs and the UI was horrendous.
I've used an X3 before, and its UI, I have to admit, was very stable. Sound quality wasn't to match the DX50, however.
I find that when you have your fixed playlist/album, all the physical buttons available on the DX50 is enough to cover your basic needs of pause/play, forward/rewind.