Of course, as with any headphone amplifier the E5 really comes into its own with headphones that are just too hard to drive for a player's internal amplification circuitry alone. I've recently got my hands on a pair of AKG studio headphones rated at 55 ohms. While they're exceptionally clean and detailed, even the beefy Samsung YP-P3 struggled to get maximum 'oomph' out of them.
With the E5 plugged in that all changes, and while the output might still be a little flat or even sterile for some tastes (these are studio 'phones, after all), it's much richer and more lively than it was, and the results with classical music are nigh on dazzling. If you're struggling to drive high impedance cans like the Sennheiser HD5x5 series or earbuds like the Klipsch Image or Sleek Audio SA6, you might want to bear this in mind; the E5 can get you decent results, without putting a huge hole in your wallet.
And this is where we come to my rule of thumb for the FiiO E5. If your current sound can be thin or brittle or even rather clinical, and you don't want to change your player or your headphones, then this little beauty might just do the trick, if only through the bass boost function. If you already get a sound you're happy with at between the 50 and 70 per cent volume mark, you still might get an improvement from the E5, but don't expect any miracles.
If, however, you're pushing the volume on your player beyond the 65 to 70 per cent mark just in order to get a decent sound, then the E5 is probably the most cost practical and cost effective way you might find of reaching your setup's full potential. You won't get the same audio quality you'd expect from, say, the Graham Slee Voyager or even a really good Cmoy amp, but then you're not paying £150 for audiophile gear, but getting something you can clip on your jacket that costs less than twenty quid. Even if you only want to use it on the road, that makes the E5 one of the best bargains I've seen so far this year.
Solid audio output for such a ludicrously cheap headphone amp. While your mileage will vary depending on your DAP/PMP and 'phones, it's well worth giving the E5 a try if you find your existing setup lacks power.