Aside from the aux switch, I don't find any benefit to the E09k. First, I can discern no difference whatsoever between the sound output of either device when used with my E17, so sound quality is unchanged between units. I realize this was expected, and I've been pleased with the performance so I wasn't looking for an increase in quality.
Second, the E09K might have a unified shutdown for the E17, but I find it pretty useless since it still doesn't sense USB input status. In other words, if using a USB source, and I shut the computer down, the E09K / E17 still stays awake. I expect my usb devices to shut down when the computer shuts down, otherwise I would be pushing off buttons and switches all over my desk!
Third, no 3.5" jack on the E09k isn't a deal breaker but it is a little inconvenient to have to find my adapter to use my Westone IEMs.
Fourth, and this is the biggie, there is no input prioritizing on the E09K. I always have my Audioengine A2 studio monitors connected to the back of the E09K, and now when I plug in my DT990s to the front headphone output, the A2s still play with my headphones. I can't understand why they would take this functionality away from the E9. Does the target audience for this device use headphones and speakers at the same time with a device like this? Sure, FiiO made it so I didn't have to pull the E17 off the dock every time I wanted an AUX input, but then they make me turn off my studio monitors every time I want to listen with headphones. Doesn't make sense to me.
TL;DR: I went back to the E9 because the E09K is more inconvenient than the E9 it replaced.