I'd recently bought a pair of Audio-Technica ATH-M50s; they sound great coming from my computer but there is a fairly noticeable decrease in sound quality when coming out of my iPod (a 5th generation Nano). Now iPods are universally known as having fairly good sound quality, but not great: they are popular for their good interface and other features, etc. One other problem I had with my iPod was that the volume Did Not Go High Enough; as I am a bit of an audiophile I wished to have more of a range of volume that I could listen to, especially for quieter orchestral tunes. Rather than spend on a new MP3 player (such as a Zune or Walkman) I decided it would be worth buying a headphone amplifier, for improving the sound and boosting the power going to the headphones.
The FiiO does this excellently. It arrived quickly and neatly packaged: there was of course a small instruction manual which, being manly, I didn't read *shifty glance across the room*; two 3.5mm (standard headphone plug size) wires of different lengths for connecting the amp to your MP3 player; a pair of stylish little clips you could attach to the FiiO if you wanted to clip it to your shirt or something (so far, I haven't used either them; I am also at a complete loss as to why there are two, as it's fairly hard to clip it to two items of clothing at once); and naturally a USB charging cable which is the standard, universal kind (does it have a proper name? You can see the socket on the picture, anyway).
So I charged it up, plugged it in and turned it on, and was extremely impressed (don't over think what I just wrote). There is a handy volume control on the side (closest to your headphone cable to turn it up, closest to the iPod connector to turn it down is how I remember) as well as three EQ settings indicated by a tiny little round light on the back of the device: the first is a bass booster (indicated by the little light turning red) which I was in awe of. Usually with bass-boosting EQ's, you get a fair amount of distortion from the device, but with the FiiO it is incredibly smooth and noiseless; if there is one issue I have with the setting, it is that it turns the treble down as well, but that's just personal preference. The second setting is treble booster (purple light) which is equally excellent; the third, I'm fairly sure, is Flat (indicated by blue). I have to say though that I leave the EQ turned off, but it depends on what kind of listening experience you prefer. You change the EQs using the on/off switch on the other side of the device (you hold the switch in position to turn it on or off, move it quickly to cycle through EQs) and if you move the switch in the opposite direction you can lock the controls; though I tend to change the volume of the tracks with the amp control, not the iPod, so I usually just leave it unlocked. The device is about the size of the 6th generation iPod Nano, but not quite as thick (about 7mm).
Other than that, I can say that the volume output of the device is mind-blowing (it goes far higher than I could possibly listen to, again without distortion), and the quality of the sound is excellent - there is the barest trace of noise when you have no music playing, but considering the price, I wouldn't consider it the slightest of problems. If I had one criticism it would be the battery life: I've been using the FiiO on bus journeys at a fairly average kind of volume, but even fully charged it only lasted about 5 hours. Whether that is good for its size or not I am unsure of, I've seen loads of mixed reviews around the web, so it's hard to make a judgement. That's for you to decide. Despite this it charges up fully in less than half an hour, so it isn't too much of a problem; I charge it using a USB plug in the corner, so it can be done quickly.
So for about £20.00, you can buy a small square that boosts the volume of your headphones significantly, improves the quality to a fair degree and provides a great listening experience. I highly recommend it. For the price, it does a fantastic job, and if you have an iPod pouch or something along those lines, chances are the FiiO will fit in there with it. And one other thing: it has a triangular blue light inside the corner bit (the silver part you can see in the picture, it's where the clips can be attached) that is absolutely radical. A great product.