I've owned the Fiio E17 for aprox. 5 months. Running 100% of the time through USB.
On my Denon 5000's i've found it delivers very well. Myself and others have compared it against MUCH more expensive amp/dac combos and the FIIO can hold it's own. It's not as bright as say a Yulong or Schiit but to me provides a more balanced sound. The built in EQ is great, I know this is sometimes looked down upon in the audiophile arena but a few small tweaks can really allow you to customize the sound to your specific liking, without degrading the overall sound quality.
In terms of a portable amp/dac this thing kicks ass, compared against the headroom bithead and it blew it away.
Obviously harder to drive headphones are where the FIIO E17 starts to struggle, but my denon's love this little guy!
Nice review. I agree - great lil' amp.