I just got a shuffle the other day and I have to say, overall I'm impressed. It definitely sounds better than any flash DAP i've ever heard before and it seems very loud for its size. The sound is virtually the same as the X5, with a tiny bit more punch to the bass. The ER6i definitely sounded better on the shuffle, whereas I prefered the X5 when using UE Super.fi 5 Pros where I felt the mid-bass hump was less exaggerated and the mids a little less fatiguing (though this is just personal taste).
My only gripe with the sound was that when using the Super.fi 5 Pro the shuffle output had noticeable amount of noise that increased as the volume was turned up. Not a huge issue, though with some older acoustic stuff, it was kinda annoying.
I also found that when playing high-bitrate VBR mp3s, I would occasionally hear very short "blip" sounds. My mp3s are fine and the sound artifacts seem to occur randomly, though infrequent enough for them to go unoticed.
I also didn't like the fact that you had to install something as bloated as itunes+quicktime for a simple flash player. It loads up an extra three processes at startup and when I tried to disable these, my computer would suddenly reboot when transferring music. I'm still not sure if this was the real cause, but I said enough is enough and uninstalled itunes and now simply use
iPod shuffle Database Builder, which as close to UMS as you're gonna get.
What impressed me most though is how convenient and well designed the whole package is. The player itself is just perfectly designed for its intended use, it just lets you play music without bothering you in anyway. I give it 8/10.