Review note: I configured my computer settings to play 96 khz music tracks ('.WAV' format on the computer, using Foobar2000), and made certain that any of the E17's settings for bass, treble, balance etc. were set to default (off). I connected the E17 to the computer as an ordinary USB DAC, so that the E17 would process the USB digital output and feed that to its own internal headphone amp. I then connected my headphone to the E17's headphone out. Since the USB cable that's used with the E17 carries only the unprocessed digital signal, and the output to the headphone uses the cable that comes attached (usually) to the headphone, there are no cable issues per se that could potentially degrade the sound. I set the computer's volume to maximum and then used the E17's volume controls to set the headphone listening volume.
I knew before I ordered the FiiO E17 that it was small, but when I removed it from the box it looked even smaller - slightly smaller than a deck of cards. One thing I really like about the E17 which is also true of the other DAC-plus-headphone-amp's I have is that it runs cool. So far I haven't detected any tendency for it to become warm laying flat on a table, where only the top and sides are able to act as heat sinks. I really hope this is a trend in personal audio, since I've bought laptop and tablet computers recently that get uncomfortably warm.
For those people who have been using the headphone jack on their desktop or laptop computers, and assuming that those computers have USB ports, they should expect better sound using the E17 instead of the computer's headphone jack. The fact that the E17 includes both a DAC and headphone amp in such a small package at a relatively low price, suggests to most audiophiles that the E17's sound would be of much less quality than the typical separate DAC's and headphone amps selling for 2 to 3 times the price of the E17.
I don't own the more expensive separates myself, but I have the Objective2 headphone amp that sells (assembled version) for more than the E17 even though the E17 is two components in one. And I don't hear anything to suggest that the E17 has lower sound quality than the Objective2, or that the E17 is less than a good upgrade to the computer's headphone jack for a bargain price. The actual improvement with my computers is a cleaner sound with a greater sense of "space" and "air" around the instruments. My experience in making that comparison taught me that I would not notice a major difference when switching from the computer's headphone jack to the E17's headphone jack. But when you listen through the E17's headphone jack for awhile and then switch back to the computer's headphone jack, that's when you notice how some of the life goes out of the sound.
An important issue to consider when purchasing audio components to improve sound quality is detail, i.e. how much additional detail will be revealed in the music tracks by the new components. It's possible that a new audio component could reveal existing distortions in the recording in a way that makes them less pleasant to listen to, and some buyers may experience that dreaded feeling of "Uh-oh, I need to buy more stuff", or "Crap - this isn't working out the way I expected". I didn't have that issue with the E17 though - the sound was more revealing but less harsh, and that seems to be a typical result of upgrading the computer's DAC and headphone amp.
I did have the Audioengine D1 DAC/amp on hand to compare to the E17, and so I set up two laptop PC's with identical installs of Foobar2000 v1.1.12 and played a few 24 bit 96 khz tracks on them, using the new B&W P3 headphone from each DAC/amp's headphone jack. There was a significant difference with the D1 having noticeably better soundstage or "space", with an impression of greater high frequency extension. Since one such property as soundstage, brightness et al can affect the perception of other sonic properties, it's not possible for me to evaluate each property separately to be sure what the absolute differences are.
I did most of my listening at what I think of as audiophile volume level, i.e. slightly on the loud side. I then ran the E17's volume up to maximum on several tracks to see if there were any limits, clipping or other distortions I could hear. The maximum volume seemed to be at least 10 db above my listening level and the sound was very clean at that volume, so I don't see any practical limits there, at least with reasonably efficient headphones. The Audioengine D1 gets all of its power from the USB whereas the FiiO E17 apparently gets its power from the internal rechargeable battery, so that may account for the E17 having greater headroom than USB-powered DAC/amps.
Looking at the FiiO E17 from the front, it has an LCD screen for browsing the menu and checking the current settings. It has up/down volume buttons, but those are digitally controlled and don't respond instantly. There are also buttons for power on/off, menu access, input type (USB, analog etc.) and 'hold'. On top are an input jack for optical/SPDIF, the 3.5 mm headphone out jack, and a recessed 'reset' button that will require a straight pin or something equivalent. On the right side is an 'LO bypass' switch that works only when docked with other FiiO products. On the bottom are a USB mini-jack, a FiiO dock connector and a 3.5 mm auxiliary (analog) input jack. One odd thing I noticed is that the E17's screen shows '96K 24bit' at all times when connected to my Windows 7 computer, regardless of the data rate of the track that's playing.
The E17's case is black-anodized aluminum and seems very solid and strong. The brushed-metal finish should be good for not showing scratches as long as the E17 isn't seriously abused. Given the quality of construction and the very nice overall appearance, I'd say they got the E17's design just right. Since the E17 has so many more features than the USB-powered DAC-plus-headphone-amps I mentioned here, and since the sound quality is slightly less, I would like to see a more expensive version of the E17 that has sound quality equal to those USB-powered DAC/amps such as the Audioengine D1 and the HRT Headstreamer. Still, the FiiO E17 is a tremendous bargain at its price point and beyond.