I used to have the E07K actually, and they are very good for the price.
The amp in it isn't amazing, but unless you have very high powered headphones that won't be an issue. The onboard DAC is very good though.
The only issue that's really voiced about it is the fact that in most cases you can't get the DAC to work with certain portable devices, and if you do it isn't easy.
My iPod classic wouldn't support the DAC at all. I managed to get it through my iPad by getting a camera connection kit with the USB port, then plugging the E07K into that, which isn't really a practical application. That is more a fault on Apple's side of things rather than Fiio's, but it's still a fault nonetheless.
If you are just intending to use it instead of a soundcard then it will be fine, and you will be very happy with it. If you have a bit more money however, then there probably are a few better options out there, but nothing as cheap as the Fiio.