OLED generally makes the color a lot more vibrant. When I switched from my iPad Pro 10.5 to the Samsung Galaxy S7+, the difference is very apparent. It looks nice to me. And I agree that it is better to switch to dark theme to improve battery life.
As to burn in, I feel it may still be a problem for Windows based system depending on one's usage. For example, if the laptop is used for work, chances is that the apps you use is not exactly full screen. The Windows taskbar will always be there. So that portion may see a burn in faster if the screen is very bright. On phones and tablets, this is less of an issue since the applications always run full screen, i.e. no fixed task bar in sight. That's my opinion.
Because of the way Windows renders some static elements, burn in for OLED screens is expected - definitely for the task bar, and more for other screen components. I expect OLED screened laptops to go for about 3 years or so, given the expected lifespan of OLED phones/tablets. It also means current generation OLED laptops might not have much resale power in future, for those who flip their laptops.
For power savings, what is important is that your theme has to be black, rather than dark. Pure black = no power, whereas any other shades of black will still have powered pixels that still affect battery life.
I am waiting to find more variety and affordable OLED monitors. So far this technology is taking a very long time to become mainstream for PC market.
As for brightness, because most OLED monitors are found on laptops/ tablets only, therefore, higher brightness is actually preferred. We don't always have to max out the brightness because 400 nits can be very bright when you use it indoors. But outdoors and depending on the lighting, it may not be enough.
Agree with this observation. Under the harsh sun, phones with 800+ nits might still struggle, so a 400 nit display beside a bright wall (some offices have full glass walls that you need the curtains for) will still be affected. And the battery life concerns are real too.
OLED is nice to look at, but it's not without penalties.