I own Druid Mk 4-08 in one of my Zu-based systems, and directly A/B'd Essence with them in my room.
I've written before here that I consider Essence the least-Zu speaker. It was a successful speaker for Zu in part because it has some characterisitcs appealing to what I'll term the hi-fi buyer rather than the music listener. Essence sounds more like the average hi-fi buyer expects a loudspeaker to sound, while still delivering the essential qualities of a Zu cross-overless design: holistic tone, high efficiency, transient speed and articulation, and excellent octave-to-octave balance.
There are two problems with the Essence that resulted in Druids remaining in my secondary system, and both pertain to the ribbon tweeter. First, because of the supertweeter's relative inefficiency, the FRD had to be dialed back for the two to be in balance. The result is a 4db loss in efficiency in Essence relative to Druid, and it's quite audible. Essence is less bursty, plays with audibly less dynamic life at a given power and generally sound comparatively constrained. Now, if your only prior reference is a less efficient crossover-based speaker, then Essence will sound lively compared to that. But against Druid, Essence sounds a trifle flat. Second, the essential tonal quality of the ribbon supertweeter is mismatched to the tone density of the FRD, and for me it is distractingly zippy and bothersome. There is audibly less unity between the FRD and the handoff above 12kHz.
So for the latter, some people aren't bothered, but then I've yet to hear a ribbon tweeter I can live with. The former issue of detuned dynamic performance to accommodate the ribbon in Essence is real for anyone who has a chance to hear both. I am specifically recommending Druid Mk4-08 over Essence, though less so the earlier versions of Druid.
Of the current Zu speakers, Superfly costs less than Essence and trounces it on dynamics, musicality, tone density and for being generally musically convincing. Superfly is a sensational speaker worth amplification of considerably greater cost. If you have a large room to load sonically or you just like a spatially bigger sound at some expense of ultimate precision and focus, Omen Def is also preferable in my mind to Essence.
Phil