You can look at the Phoronix benchmarks results from earlier this month. While they are certainly usable, the performance is still quite a bit lower than mid range AMD and Nvidia cards in most games.

You want to use the latest kernel and mesa versions (preferably git or release candidate versions). The new Xe kernel module allegedly doesn't support hardware accelerated video decoding so you'll need to stick with i915. If Intel's new GPUs dont have the hardware acceleration issue it would be a no-brainer to buy once the performance issues are ironed out.

Ill say promising. Mesa recently landed sparse support so that should make a lot more games playable now, perf is decent but there are for sure still bugs, for instance for me and a couple others, gamescope doesn't work right

With my 770, I mostly do desktop use and I play some Northgard, which works perfectly fine (1440p ultra settings)

friend has one, he's happy with it. It has mesa drivers so it should work great

Performance is worse than alternatives, and there are some features that work on Windows but they have said they won't support them on Linux. So, AMD is still unrivaled on Linux.