I ended up buying a pair. My Stax energizer is sitting on my desk anyways and I thought it'd be nice to have an electrostat that is suitable for more casual daily use (my 404 seems too fragile for that and is odd to wear).
To be honest, I was a bit surprised by the price: in Germany, they come in at 695€, together with the amp they would be well above 1k. Build quality is Stax-typical, they feel extremely cheap due to being lightweight, however construction generally seems reasonably good (metal yokes to hold the cups, plastic feels ok). The lack of headband padding is only a minor issue since they are so lightweight. One problem for people with large ears is going to be the small size of the ear pads, my ears fit just barely, but comfort is ok overall and I get a decent seal even with my glasses on.
As for the sound, my only point of comparison is the 404: they are a bit brighter overall, but the Stax-typical upper-mid bump appears smaller to my ears. Generally they are bright headphones (strong upper treble) but I find them a bit less sharp than the current Lambda series. The bass is similar to the 404 and therefore a bit more pronounced than on the current Lambdas. Hoping to see more measurements in the future because I think they might be one of the more neutral Stax in the current lineup. Subjectively speaking, I always got a weird imaging experience on Lambda earspeakers, the X1 doesn't quite do the same thing.
I think they are quite exciting as entry-level electrostats for those interested in what electrostatic headphones can sound like (not magic, that is for sure).
Obviously, they'd profit from EQ, but I haven't looked into that yet.