"FYI you don't have to run games at 120 / 144 Hz to get the advantage. Even on a computer that meets minimum spec, asynchronous re-projection will kick on as low as 60 FPS and make it feel like 120 Hz. The advantage of the higher refresh rate is smoother motion for not only high end machines but low end machines as well.
In addition the Index has Pixel persistence a fraction of what the Rift or Rift S has, meaning any motion, especially fast motion, will be clearer on the Index.
The Index is also the first VR headset to feature dual element lenses. This reduces chromatic aberration, improves perceived PPD, and increases FOV. Lenses can have a big impact on how sharp the display appears. Valve is claiming these lenses provide a clear vision across the entire FOV, whereas your typical lens may only provide optimal clarity in the center of your view in a small sweet spot.
From the early impressions I've seen, the Index provides a very good visual experience"