Pros: Decent audio performance, great form factor
Cons: Annoying high pitch whine while charging sometimes, didn't feel too durable and mine broke.
The Fiio E6, in all honesty, fulfilled basically my portable amp needs. It has more voltage output than my portable media players. It fit very nicely in my pocket. From a portable usability standpoint, it was great.
My main issues were from attempting to use it as a temporary desktop amp. One thing is that, although it can charge and play at the same time, it was obviously not designed to do so. There would be common high pitched whines in the music when it did this. Furthermore, the battery charges at seemingly random points when it is left plugged in and would wear out the battery over time. Although it was usable as a stopgap desktop amp, it was very poor at doing that job.
My other issue is that it was not durable. I'm pretty careful with my audio equipment and my E6 was mostly used in a desktop environment because I wanted a cheap stopgap. At some point in time, the switch to turn it on and toggle the equalizer broke -- it wouldn't toggle anymore, even though it still moved. My E6 was stuck in the off position. I could have opened it up and attempted to fix it, but it didn't really seem worth my time.
When my E6 worked, it was good on the go. Just don't expect it to be a good choice in a desktop situation or to have great build quality.