Been using it for about a month now. Definitely an upgrade over X16 (even with volume matched to ~ 4.1 V).
Though I prefer the NOS mode in both X16 and X18 in conjunction with external software upsampling.
I was in slight hesitation between this and X16 Pro. But then a significant part why X26 Pro costs more (other than using dual ES9038 Pro) is because of its USB receiver and PLL treatment (using K2 oscillators). And Gustard's custom oversampling filters which didn't look too different than ESS' built in filters.
Anyways, since I have Gustard U18 USB interface, I didn't see the need for X26 Pro.
Also, just in case this matters from design standpoint - X18 uses ESS' recommended way of I/V conversion of using separate opamps per channel and then summing up.
Read page 12 from
https://www.esstech.com/wp-content/uploads/2021/03/ES9038PRO-Datasheet-v3.7.pdf
How much of this translates to perceptible effect - I don't know