Excellent, very detailed review! However, considering the weight of the pros and the cons, especially considering the affordability, I would have given it a 4.5, if not a full 5 stars. I have one, and I enjoy it thoroughly.

Thanks! It's clearly a positive review, but I'm leaving room for updates. Also it's only one data point. I'm somewhat surprised that no one else has left an extended review here, given the uniqueness of the product and the time that it has been out.

Radsone hasn't released a firmware update since July. Wishful thinking has me hoping that they're working on something big...

Firmware v1.4.2 released 2018-09-17: https://www.radsone.com/earstudio
USB DAC 44.1 KHz support for Windows is a nice addition; you can omit the 44.1->48 KHz resampling, which would apply to all Redbook CD audio files.
dspGuru FAQ on resampling: https://dspguru.com/dsp/faqs/multirate/resampling/
24-bit support on macOS gives the user some extra headroom for non-destructive software volume control for 16-bit file playback on your mac.

Any idea if the EQ profile is stored in the ES100 and available if streaming from a different source device later? Somehow your description of this as a killer feature seems to suggest that might be the case, but in such a small device... I don't know...

@abm0 Eh, I replied to the main product page, not the review comments. In any case, you should see the other message. In short, the answer to your question is in the app itself. Look at the screenshot I tacked on at the bottom of my review.