So! My S arrived this afternoon and I had a quick look at a few things. Overall?
Yeah as @Editer says it’s a pretty nice improvement, far better than the modest spec bump would indicate. The LCD panel pixel layout/density and new lenses make a decent difference!
Everything is sharper and more vibrant. Text is more legible, and you can resolve objects a bit farther into the distance. CV1 was always like looking through thin gauze, this is like the gauze has been ripped away. The contrast seems fine, as DennyA said the overall improvements make any real difference there may (or may not) be basically moot.
The ‘screen door’ is improved but absolutely still there (more noticeable at mid-to-far range than close up). Except now it’s a more pleasing black/white grid, and I don’t notice the diamond RGB subpixel layout like I do in Rift. I’m sure there are proper comparison pics, but it’s kinda like the difference between these:

The old ‘god ray’ lens effects that eminate from bright areas are mostly gone on the S.
It’s comfortable. Feels plushy on the face, the halo headband has a great tightening dial mechanism, there’s almost no nose gap. The tether cable is thicker and heavier though which is a little annoying.
There is a button so you can move the lenses forwards, to accomodate glasses - I have not tried it with glasses though. The software-only IPD adjustment means it may not work well for the 20% of people who fall far outside the ‘average’ IPD range. I can’t test it as I’m right near the middle at 64.
The audio is much better than I expected. Like wow, I do not miss having things sitting on my ears at all. :)
The pass-through is pretty cool, it can activate automatically when you go oustide guardian boundary, or you can double-tap Oculus button to bring it up. The whole set-up is slicker using it.
The new controllers are definitely cheaper. The sticks seem smaller and a bit looser, the buttons a bit clackier. Same nice subtle rumbles though, and they’re not bad - just not quite as solid. I don’t think there’s a capacitive touch surface like Rift has, though all the buttons are still capacitive and register touch as well as press. The only game I ever played that used that capacitive surface feature was X Rebirth VR so maybe it’s not a huge loss.
To try the Insight tracking I played the rogue-lite archery game In Death for a while and it was pretty flawless. Drawing the hand back near the neck over and over didn’t seem to phase it.
All in all it’s a nicer little refresh than I was expecting based on reading specs alone, quite pleased. Now to figure out what to do with all those free usb ports!