I've owned a CV1, Odyssey + and now the Valve Index which came in a few days ago. I'm only three days in, but I'll summarize my feedback below.
Resolution: A great step up from anything else i've worn. On paper, the HP Reverb has it beat but this is a noticeable step up from the CV1 and a clarity improvement over the Odyssey which had a bit of a smeared effect due to how they tried to remove screen door effect. I'm all about crisp, clear picture and the Index delivers here. If you look, you can still see a bit of screen door but all the words on the dash are clear and I can read everything (well, except for ACC dashes but that's the UE4 engine). Colors are very good and seem more vibrant than the CV1 but not as overexposed as the OD+. FOV is improved over the rift, but it's not a massive jump. I noticed it mostly in the vertical space.
Sound: This is a game changer for me. Sound is a big part and although the CV1 had passable audio, it never was great and the microphone was terrible. The speakers on the Index sit just off your ear and have great range but also allow a little sound to come through at low levels so your wife/kids don't scare the hell out of you.
Comfort: Puts a little pressure on the upper bridge of my nose, but i'm still playing with the fit. I like the lens adjustability knob which lets you bring it closer/farther away from your eyes for maximum FOV. Sits well on the head and I can lean it against my race seat without any issues. It is warm, but so was the CV1 so it's hard for me to tell if its worse.
Concerns: I think there are two concerns that I have at the moment. 1) You need some good hardware to run this thing. I have an i7-7700k and 1080 and I need a better GPU at the moment. It definitely requires top end hardware unless you want to run at minimal settings. 2) The software in WMR for motion smoothing (ASW equivalent) is good but not as good as Oculus. If you put the light houses on my rig under big jolts/bumps it throws off the headset tracking and causing a temporary gray screne and the smoothing just isn't as good when you drop frames. Will it get better? Maybe...but it's not as good as Oculus.
EDIT: I'll add a couple more gripes. I have triple screens as well and switch back and forth depending upon the sim. It's a pain to switch now because this headset used a DP connection. Second, I wish there was an easier way to reduce audio volume. Oculus had a handy remote...no such easy option here.