Pros: Excellent sound quality for the price; sturdy build quality; great mids; good carrying pouch
Cons: Needs amplification; slightly sibilant treble
The ES800 are the kind of IEMs you won't like how they sound out of the box: they sound thin and aggressive, with piercing mids and highs. Then you will question the design: the hideous "gold drop" on the back, the flat cable, the enormous cable splitter. The tips are not so good either. So, there is a chance that, for 3 or 4 dollars, you will give it to that annoying cousin, or forget about it in the bottom of a drawer. Don't! They just need some love and care. And about 100 hours of burn-in.
First, take those ugly-looking yellow stickers from the back. There is a lovely smooth surface underneath! Then, stick-em some Sony hybrid tips. And, at last, give them some current. They need current, and power. Then you will start to fall in love with them, and to forget about the flat cable and the big-as-hell metal splitter.
Yes, they never stop sounding too thin on your smartphone - and my Lumia 925 has no problem driving any other IEMs but these. You will need to amplify it. I use the Cmoy amp with great results, but I guess your tiny Fiio will do the trick.
As for the build quality,once you pass through the annoyance of the flat cable, it is really good. Metal housings and one of the best L-shaped 3.5 jack I've ever used! 3.5 dollars? WTF? Give them a try!