As another steam deck user (~1 year of 1-4h daily usage):

performance/compatibility:

• modern AAA games can be played but expect low settings and 30FPS. I don't play many of those so it's not a problem
• indies work, with good battery life
• old PC games work well, e.g. Fallout 2 on Steam out of the box - the trackpads are important here to replace mouse. Don't expect to play a micro-heavy RTS though.
• AAA games from 2012-2020 (~PS4 generation) work with good enough battery life for my commute (~1+1h)
• setup of emulators is trivial with EmuDeck
• switch emulation works (a recent yuzu update bumped performance to 'as good as switch' for almost everything.
• PS2, PSP, Wii (and everything older) emulation works, but don't expect PS3 to work
• most multiplayer games with anticheat don't work
• modding windows games (outside those with Steam Workshop support) is impractical, you need to go into desktop mode and mess with the particular proton 'bottle' for that game
• adding third-party games is easy (add the game's binary to steam and tell it which Proton to use)

ergonomics/size:

• it's big, not laptop big, but a backpack is the most practical way to carry it (I carry it with my work laptop)
• It's really comfortable to hold - personally it's more comfortable than my Logitech F710 (controller) - but I have big hands

reliability/stability:

• no SW issues so far, good cadence of updates
• no analog stick drift so far
• no measurable battery degradation so far

hardware:

• not the easiest device to take apart (e.g. if you want to upgrade the SSD)

other:

• a good big uSD card may be preferable to buying the most expensive model, I have e.g. Witcher 3 on an uSD card and loading is (subjectively) fast enough.
• steam has per-game controller schemes which you can download from other users, this is especially convenient for strategy games where there's no 'common scheme'
• you can set screen refresh rate and FPS limit per-game, e.g. for turn-based games I go way down with FPS (~20) to save battery
• people complain about the screen, IMO it's comparable with any 'normal', non-OLED monitor

There's also a considerable dev community around Steam Deck, e.g. decky-loader for plugins, and already mentioned EmuDeck.