Non-sound related aspects

Fitment:
Fit is perhaps the most interesting part about the EX1K. When in the ears, the driver is almost in line with the ear canal. What this looks like to an outside observer, they resemble "elephant ears" in a sense. It's perhaps a little weird, but I've seen worse. However, the fitment and design means that these are very comfortable.
Daily usability:
I really wouldn't use these in public. Apart from fitment, these have extremely low isolation, meaning that a lot of the nuance will get lost with the outside noise. While they aren't like the open back offerings from Audeze, these barely isolate. I suppose if you're fine with that and the odd fit, then these would be a decent daily driver due to the comfort. I just use them at home.
Build:
The IEMs themselves are structurally sound. I believe Sony used magnesium for the chassis, which is light and durable, while having beneficial acoustic properties. However, the outside paint is easily damaged. My units suffer from their paint and markings chipping off. The filters are also made of a foam which will inevitably degrade. Sony doesn't make replacement filters to my knowledge, though I've seen people get creative with DIY solutions. The EX1Ks use a proprietary connector, which you will need to buy an adapter if you wish to use standard cables with it. The cable (RK-EX1000) isn't very impressive. The cable is in my opinion not much better than many $20 iems. At least it isn't too microphonic.

Sound

Bass:
Bass is very impressive. The EX1K delivers one of the best bass experiences you will find far beyond it's price. Sony knows its dynamic drivers after all. The EX1K has exceptionally detailed and impactful bass. Not much else I can say, as it's just that good. It's not bass head level, however, with some eq...
Midrange:
Mids are similar to the bass, amazing, almost without complaint. It's rather flat, so nothing particularly stands out.
Treble:
I consider myself more tolerant to treble than most. Headphones like the Arya are to my taste. The treble is arguably the biggest "X factor" of the EX1K. Notably, the dip starting at 2/3k and the two peaks in the mid treble. Starting with the dip, it gives certain instruments and vocals a bit less presence. If accounted for, "z" and other sibilants will be a bit more apparent. You may or may not like this, though a bit of eq will help female vocals quite a lot. In my opinion, the peaks in the treble aren't a problem. The treble of the EX1K really contributes to its detail pickup after all. My another issue with the EX1K is the air. Simply put, there isn't enough, leading to a slightly congested feeling. A high shelf fixes this.
Subjectives
The EX1K possesses very strong subjective aspects in all areas. Detail retrieval is pretty good, especially in 2024. It’s capable of almost anything, without being too force-fed in nature. Soundstage is dare I say, the best within its used price range. I personally don’t consider imaging too important, but it’s accurate, and I have no complaints.
Equalisation
The 16mm driver of the EX1K is very tolerant to eq. With eq, this thing can be whatever you want. The EX1K with a bass shelf is an experience. I would highly recommend using this driver to its full potential.

Conclusion
To conclude, the EX1K is a very unique iem which has some outstanding qualities which keep it in the conversation even in 2024s competitive market. Admittedly, it's quirks and features might be a dealbreaker, notably the non sound aspects in addition to the abnormal treble. Depending on your situation and requirements, the EX1K could bring you very close to "endgame". To me, the EX1K's sound is excellent, perhaps the best in some cases, but it's other traits bring it down.

You probably know what Etymotic and the ER4XR are, so I won't introduce them for brevity, as I'd like to keep this review short and to the point. The ER4XR is currently $300 USD in Australia.

User experience
The triple and double flange Etymotic experience can be daunting for those who have not experienced deep insertion. For the first few days, it hurt, and got better the more I wore them. Your results may vary though, as I have small ear canals. I would recommend sleeping with them to get used to the fit. The upper portion of the cable is microphonic. although the cable overall passes the roadie wrap test and feels nice, though has a more 'studio' type build as opposed to a 'luxury' build. No need to comment on isolation either.

Bass
Bass is not abundant despite the XR moniker. Although, it is clean and isn't lacking. If EQ is applied, then the ER4XR can be bought to a more 'fun' level, although lacks a certain satisfaction/impact due to the single ba. Genres like EDM still perform well on these; they're studio monitors for professionals, after all.

Mids
The Mids remind me of the 6xx series to a certain extent. Clean, accurate and intimate. Vocals and acoustic music shine on this. If you like the 6xx series of headphones, you are likely to enjoy the mids of the ER4XR.

Treble
The ER4XR is my reference for treble in iems. It's not perfect, but it's very close. Minor adjustments in the air region and you get the best treble out of anything really, although perhaps the Shure KSEs may dethrone it, but I haven't heard them yet.

Technicalities
Not a big fan of describing things that can't be quantified into numbers, but alas, here's what I think.
Soundstage is intimate. Could be opened up, but in my use case, which is in public environments, that doesn't matter. I just need something to force feed detail into my ears even with external noises, which this does very well. These are very detailed to the point which you cannot escape it. All the nuances of the music are presented to you plainly, a trait the original target audience of Etymotic products are very likely to seek. Imaging is accurate, though I do not care about it. Imaging and soundstage aren't the focus of the iems, and that isn't a concern to me, but it might be to you.

Other notes/Things to consider
Etymotics have proprietary cables, which require the usage of adapters in order to be used with bt earhooks. It's not too big of a problem, though it will mean an additional bit of money spent. Etymotics have some pretty expensive first party eartips, and the replaceable filters will also cost you.

Conclusion
Your satisfaction with these iems will really depend on what you're looking for. A non intrusive fit? Probably not. Isolation? Absolutely. Planar bass? No. Amazing treble? Yes. I could go on. These just happen to align extremely well with my use case, which is for listening outside of home as a daily driver iem which I can listen to in public and enjoy high levels of details without interruption from noise. Nothing else really provides what these provide, and unless another deep insertion iem that does everything better comes out at a similar or lower price, I'll be daily driving these until my tastes or priorities change.