I knew which negatives to expect from these headphones out of the box from reading all of the reviews and forum posts: recessed sub-bass, apparent lack of extension, present but shy highs, and a mediocre soundstage for such an open can. However, you can notice that most of those issues are with the sound signature (frequency response), not the more complex presentation qualities such as tibre, imaging, decay, etc, for which electrostatics are typically superior to planars or standard dynamic drivers due to their sheer speed and resolution.
Fortunately, I'm not afraid to use digital equalization like many headphone folks out there are (for unfounded reasons if you ask me). I also knew that these cans can take an EQ like a champ, so using Oratory1990's parametric Harman curve EQ settings as a starting point, I got to tinkering. The results, after proper burn-in of course, is a headphone that makes every other headphone I've heard (HD600, EL-8c, DT990 Pro, NAD HP50, and a few others) sound rather muddy and dull. Sure, they don't have the raw bass slam that my Audeze planars have, but on every other category, I much prefer the ESP/95X as my daily drivers.
Their huge drivers provide an enveloping, clearly layered, and airy sphere of sound around you with excellent imaging. Meanwhile, the EQ makes the cans to sound more natural and well balanced, with deep textured bass extending well below 30 Hz and crisp highs that have shimmer without being fatiguing. The midrange shout has been completely tamed.
In terms of build I won't say much. Plastic everywhere, and certainly not premium-feeling in any way, but the quality of the fits and moving components is indicative of a properly designed product made to last a long time if taken care of. After all, the goal of the engineers at Koss was to create a light headphone that could we worn for hours at a time, something which they've certainly succeeded at.
TLDR: With some EQ you can get the advantages that electrostatics can offer without any of the drawbacks in sound signature.
Side note: after several months of use, the ear pad foam has broken in a bit, becoming softer and reducing the crunching sound when moving your head that was a quite distracting when they were brand new.