Personally, a solid 8/10. Steam is probably the best experience if you are playing Steam games but there are also other third party launches that can handle Epic games and GOG and others.

Some games run natively but most will be using Wine, or Steam's implementation Proton. If you have issues, you can check out tinker steps on https://www.protondb.com (and also check there before buying a game to see if others have been running it fine).

The vast majority of games work out of the box, some need some tweaks, and I don't think I've come across any that I wanted to play that don't work at all.

Steam Deck and Steam's investment in linux has really been a game changer in this space.