This is it, the Westone UM3X. In my hands (or should I say... my ears!). Thanks to @mark2410 for loaning me his. I've only managed to listen to it a couple of hours, but I am deeply impressed by it. I will eventually compare it with my IE 8 (of which has close to a thousand hours on it). Both offers completely different sound signature. It is just a matter of pinpointing your preference. The UM3X is likely to appeal more towards owners of other balanced armature IEMs like the Shure SE530, but I only have the IE 8 here to compare with.
My tip of choice are the Klipsch gels that can be found on their Custom and Image series. They fit the UM3X perfectly, has the same bore diameter and are very very comfortable (at least for me). Beware that the sound signature will be dependent on your choice of tips. Impressions and the following review (next week?) will be my own, so please do not flame me for stating my opinions. I also would not comment about multi-flange ear tips, as I do not find them as comfortable. This is due to my shallow ear canals (one reason why the IE 8 with default single flange silicon tips are perfect for me).
The UM3X fits well in my ears and I never had a problem with seal. Isolation is better than the IE 8 (with standard single-flange silicon tips), though this could be both positive and negative. The lack of isolation with the IE 8 is one of the reason of its airiness and vast soundstaging. They seem to be as comfortable as the Klipsch Customs (easily the most comfortable IEMs I have ever had the pleasure of sticking inside my ear canals). Also, the UM3X sits flush inside the ears, so you will not have any issues sleeping with them. I wouldn't recommend sleeping with it though.
As I only have had a couple of hours with them, I won't comment much about the sound quality. But they do sound good, and even more importantly, do they sound £120 (price difference in the UK) better than the IE 8? I can't say for sure yet. They are a bit more forward than the IE 8, particularly in the mids. The bass is just a bit behind the IE 8, and has some impact, though not quite there. Just a note here that I am no bass head and I never found the bass on the IE 8 (mininum dial) to be overwhelming. Because the UM3X are fitted with balanced armatures, bass impact will definitely depends on seal and tip type.
Westone's flagship is definitely the more neutral (and thus 'boring') phone of the two, but right now I am leaning more towards the IE 8 in terms of sound preference because I just find it to be more musical. Having said that I can't yet reach a conclusion (especially with so few hours with it). All I can is for sure is that the UM3X are very good IEMs (there are no such thing as poor performing high-end IEMs - they all depends on your preference), but they also cost significantly more than the IE 8 (in the UK - in the US and some other countries the pricing gap is smaller).
Look out for my review sometime in the next week or two once I have a bit more time with it, and then another one for Denon's flagship AH-C710.