I've owned the 5909s for several months now and would like to make a few comments.
First, the 5909 is primarily a wireless & ANC headphone and, as such, sound quality is actually very good/excellent, at least compared to all other BT/ANC headphones out there (and I've tried and owned most, if not all, of the more expensive offerings by the competition). The fact that in passive mode the 5909 delivers very good sound is an added bonus as competing products tend to perform very poorly when used passively.
Unlike amirm, I think build quality is actually very good and quite a step up from the K371 (only B&O's H95 and Apple's APM offer such good quality materials. The design, though, is still not my cup of tea, but this is a solidly built headphone and, crucially, one of the most comfortable ones out there. And although not often mentioned in posts or reviews, the 5909 travel case is ideal for its intended purpose – this isn't always the case even with some premium BT/ANC headphones. Add to that that all accessories (cables, adapters, case) are truly premium, the cables themselves being also quite suple, best in class.
With regards to the ongoing discussion of soundstage/headstage, it's interesting that most people reviewing the 5909 (me included) have mentioned the remarkable wide soundstage – again, this is compared to competing BT/ANC headsets in the market. Frankly, this aspect caught me by surprise and I believe the 5909's FR is largely responsible for this. This is also truly the first headphone I've tried where the music does not sound compressed or digitised in BT mode, and I'm using 256kbps AAC files both on my MBP and iPhone.
So, taking all individual elements of the 5909, and taken as a whole package, I don't actually think this is outrageously expensive headphone.
Things that need improving: a) ML must fix the unacceptable bass boost when ANC is enabled in wireless mode; b) head-on detection is still hit and miss even after latest and only FW update; and c) 16 volume steps on iOS/macOS is unacceptable for a product of this nature (with masters/recordings of the last 25-30 years a single volume step can sound too loud or too quiet).