How is mobile VR?

Been using desktop VR for years. Thinking of upgrading my phone to a Samsung Note 8 or Pixel 2. If I get one, I'll probably also get the Gear VR/Daydream.

How is mobile VR? Obviously I assume it won't be able to render anything near what my desktop can, but I really don't see much written out there about them other than reviewer's subjective experiences of them.

I know each has their own 'app store'. How locked down is each platform? Can either system run sideloaded applications? Are Gear apps specific to Samsung devices or are they just standard Android apks?

Are VR apps rendered at native resolution or are they upsampled from a lower resolution? I see mobile versions of video files for VR are generally smaller in resolution and bit rate, why is this? It's just prerendered video, it doesn't seem like it should make a difference.

Also, besides video, what are the most useful applications, and what type of applications exist for the platform? For example on desktop there's obviously gaming, but also cinema apps, social experiences like Bigscreen or Altspace, picture in picture style overlays like OVR, and just ways to interact with your desktop environment. Does mobile VR have all this, or are there things unique to the platform?