The TWS600 are the first true wireless in-ear monitors designed by Hifiman.
The meaning of “true wireless” is that, while normally a Bluetooth earphone has wire connecting the left and right channels (and, frequently, the segment housing the wire also contains the BT receiver), the TWS600 have no wire and they can be connected independently to the smartphone (or other source).
Inside each earpiece, there is an independent BT receiver, DAC/amp circuitry and battery.
The TWS600 come with a dock system that works both as a carrying case, battery charger, power bank. It’s very portable and convenient, and dramatically increases the battery life of the TWS600 during long trips: while the TWS600 battery life lasts about 5 hours of continuous usage (they are offically rated to last 5.5h), putting them in the dock for charging, will let thme last over 30 hours more (wthout connecting to wall electricity).
The TWS600 come with a reasonably wide selection of eartips: triflangs, and differently sized biflanges, but no foam tips. Generally, foam tips are the most comfortable, but with dynamic driver IEMs (such as the TWS600), my favourite eartips for sound are large biflanges, which manage to strike the best balance of details and low end.
 

 
Sound

The TWS600 are based on Hifiman RE600 dynamic driver and mainly tuned towards a spacious, trebly balance. Their stronger points is the open presentation and high level of detail.
The music flow clearly across the spectrum, the bass is clean and fast, although lacking some depth and impact. The sense of space is quite good (especially for the price range), as well as the soundstage size, courtesy of the RE600 drivers.
The mids are a bit recessed, giving the perception of certain sounds being farther away from the listener and allowing the treble to shine more. The treble focus, as often happens with bright earphones, translates in energy that maintains the sense of pacing and engagement.
Complex music passages never sound muddied, although there the overall sense of clarity and detail is a bit inferior to that of the RE600.
Despite the middle treble focus, the TWS600 are quite smooth and not sibilant, probably a characteristic of the dynamic driver technology.
The TWS600 sound very good with with electronic music, full bodied music and music with high production values, but for other material (such as old music, poorly produced rock, low fi stuff, old jazz recordings) they require strong equalization centered in low to mid midrange (3-6 dB), especially around 300-2000 Hz, which would make vocals closer and meatier, adding presence and some volume to them.
 

 
Bottom Line

These are a very interesting product, especially technology-wise, given their ability to run independently (each playing in mono, or together in stereo), and for the price range they manage to pack many advantages: clear sound quality, isolation, good battery life, zero wires at all.
The differences between the TWS600 and the RE600 could be caused by the Bluetooth technology and the implied constraints (such as power saving in the behavior of the earpiece), compared to those provided by the usage of an external amp wired headphones (such DAC / amp circuitry present inside smartphones / digital audio players). Or perhaps, such differences could depend on fit, which is obviously different for the TWS600, given their size.
They have several desirable qualities (not least, the useful dock, which does triple duty of being a snug carrying case, charger and also power bank) but one must be aware of the main compromise: the lack of bass and midrange fullness.