Smoother isn't better IMO and I definitely find the E18 to be an upgrade with respect E07K and E17. It's sharper and drier maybe, but that's the kind of sound I prefer. Some people mistakes the tightness on the bass with warmth. The E18 is much more precise in the bass than the laters as well. I suppose it's a matter of opinion but I A/B'ed the E18 with the O2/ODAC combo in the Q701 and the difference were not that big. I can perhaps say that the DAC on the E18 is indeed slightly warmer that the ODAC, compared from the aux output.
A piece of equipment that is definitely worth every single cent.
EDIT:
What I found slightly disappointing was the build. The chromed bezel is a bit loose and I expected an improvement with respect the E07K that feels sturdy and solid. This is probably not the case in the E18.
Regarding usability, this is honestly ridiculously simple to use. I've never had a single problem with it. It has a lot of functionality, but that is an extra and definitely not a flaw. In fact, I greatly appreciate not having the LED screen and the digital volume. You can operate the device without even looking to it.
Finally, after the upgrade that Fiio has already announced for January 15th, this device is going to be even better. I expect them to fix the little quality control issues they are having at this point. I will definitely be selling mine to get the new version.