Audio quality: Much better than the original DX7, haha. Compared to my TX802, it's pretty close. I've A/B'd recordings made with my 802, Hexter nails its sound in most patches.
UI: Minimal. Fine by me, I'm after sound and facility.
Ease of use: Straightforward operation. UI is divided into three tabs and an editor page.
Features: Remote editing turned out to be very helpful. Imports a lot of DX/TX patch formats. Vast number of patches available. Easy to configure multiple instances when used with the Ghostess DSSI host, much cheaper than buying a used TX802 or TX816.
Downers: Not easy to substitute default bank. Mono-only output. Fixed MIDI receive channel (unless used with Ghostess).
I wrote a lot of music for a pair of TX802s, with a lot of custom patches in Voyetra's Patchmaster and Sideman formats. Hexter happily imported my original patches and recreates them with high accuracy. I use Hexter with Ghostess, freely routing its output to other JACK clients, e.g the CALF reverb plugin or an EQ from linuxDSP.
The last time I checked for a TX816 on eBay the asking price was $999, US dollars. Hexter sounds better, is easier to use, has more polyphony and no batteries, and costs $999 less. It really is a great deal.