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:

hardware:

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.