Personally I'll always prefer mouse+keyboard, but it's also fun to play games in the living room.
I want one, but I'm still having trouble wrapping my head around what I would actually do.
Huge range of games with a wide variety of inputs. I don't want to admit to you what my weekly playtime average looks like since receiving the Deck, but it's high.
Vampire Survivors is my most highest played one based on hours though =) You can play it with the sound and most of your brain off, so you can still listen to what's going on in the TV show you're "watching".
GTA V runs perfectly on it, I played through Quantum Break. And as soon as I get Heroic Launcher (Epic + Gog games) working, I'll start with Witcher 3 and CP2077.
Honestly, its more that it feels like a new input type entirely in a class of its own. I find I use it a lot to mix and match gamepad controls with keyboard shortcuts and mouse click-drag, sometimes all within the same game.
Noita has gamepad controls, but a lot of the interface works a lot better with mouse+keyboard. Deck lets me use gamepad controls for when my character is interacting with the environment, and use mouse+keyboard controls when I open my inventory. This wouldn't be realistic on another device, because the environment/world continues in the background even while you have your inventory open, so you need to be able to switch immediately from inventory management back to world-interaction.
Most games are like this: they rely heavily on one input, but being able to make use of the other inputs simultaneously gives you a ton of options.
The one class of game that I really enjoy but haven't tried yet is heavily-keyboard-focused games like some traditional roguelikes. Some, like Brogue, have a small enough control set that it would be pretty easy to map all the main keys, and supplement with mouse input for eg inventory selection. Many require more keys and don't allow mouse input, however. Cataclysm: Dark Days Ahead has something like FIFO keyboard shortcut for inventory items, and uses a-z plus several symbols; that would be pretty painful on Deck. I haven't played that in years though, so maybe would be better today. I've been seriously meaning to try Cogmind or Caves of Qud on the Deck, though.
