Pros: Tons of power, not much coloration, works well with the E17, looks great
Cons: Distortion at higher gains
I have used this amp for about a year now, and I must say it has served me well.
Through many different headphones, the FiiO E09K hasn't skipped a beat, and has driven everything from high power, low impedance to high sensitivity to 250ohm headphones. It really is a great amp to start with, or to buy if you're on a budget, simply because it can handle everything you throw at it.
This amp is probably one of the best I've had the pleasure of owning in terms of overall build quality. It has a very sturdy feel to it, with its solid enclosure and steady dial. Never once did I have to worry about breaking it, or someone else breaking it. It also takes up very little space on my desk, which is certainly a plus if you're working in a smaller area. It also looks fantastic!
The E09K seems to simply give you pure, unaltered sound. It doesn't color much, and you'll probably end up hearing more of the traits of your headphones than that of your amp. Now, this might be a pro or a con for you. It was a con for me, since I'm not mixing, and I like my audio smoothed down a bit. Also, there is some audible distortion at higher gains, which was a bit of a pain, but not too much of a problem most of the time. It was really only in select situations when I ran into problems.
The ability to dock a portable FiiO DAC is very neat. I do own a E17, however, I didn't often use it with the FiiO. It actually seemed to degrade the audio quality, especially if you're using its equalizer. I can't say I'm a big fan of the combo, however, it was very easy to set up, and the two units worked flawlessly together. If you need a portable amp, I would highly recommend it just because it's nice to have the option of using optical.
This is a really good amp for the price. I'm actually going to be selling it soon to get a tube (but seriously FiiO, it's not you, it's me). If you want low amounts of coloration and an amp that will serve you well even as you buy better headphones, this amp is amazing.