I wanted to give my quick thoughts on theater mode so far because that's been my most anticipated use case for this device. Thank goodness for OLED displays!
The experience is like having my own personal movie theater, and the quality of the display and lenses definitely give it that "high-end projector" sorta look to it. It's not as crisp as a TV, but that's primarily due to the edge-to-edge clarity not being the best on this headset. So far, I love it, but with some little caveats.
My big complaint about theater mode is, weirdly enough, I wish I could make the screen smaller. Even at the smallest size, it really is quite huge! I've only tried Returnal on it, but the massive size of that virtual display made it difficult to get a quick read on my HUD elements because they were so far away in the corners, and I also lost track of enemies because my peripheral vision was so overwhelmed by being so close to the action. To take the "personal movie theater" analogy further, playing Returnal was like sitting in the first three rows at an IMAX showing. I imagine games with a more zoomed out camera or less HUD elements will play great, but shooters or close quarters third-person games will probably feel a bit strange.
Another thing I noticed was how damn bright everything could get. The black levels were wild, but the brightness! Wow! Those projectiles look like fireworks. And funnily enough, the act of using the melee ability in Returnal is so bright that it actually make other in-game elements difficult to see.
The only other weird thing with theater mode is the screen never changes size or orientation. If you stand back or sit back in your chair, you don't get further away from the screen like you might expect. This makes sense, but was very trippy at first because you really do get lost in the experience and feel like you're in your own private movie screening. It'd be nice if you could change that in the settings menu.
I'm eager to play more flat games on this thing, honestly. It's cool, even if my only example so far was a little disorienting.