Had my Deck for a couple of weeks now and it's changed the way I game. I've not played anything on my gaming rig (good spec, 3080, 64GB, i7 etc.) since the Deck arrived.
I've completed Control on it which was a blast and suits the hand-held form factor very well. I've started playing through Halo MCC which I thought I would hate with a gamepad but I've actually got used to it. I played through GTA5 story mode again just for fun and I've also logged a ton of hours in a bunch of indie games and emulators.
I've 3d printed a stand / dock so I can play on the TV, and the SteamClip system for attaching batteries, USB drives, etc to the back.
I've got a 256GB version and I've already filled four 512GB MicroSDs with games. This thing is going to be a godsend next week when I'm in Poland at my mother-in-law's for 2 weeks.
The only thing I don't like about it is that the screen sucks. Looking at it next to my son's OLED Switch there is no contest. The one on the Steamdeck is really poor in comparison. Wondering whether someone will rustle up an OLED screen upgrade for it.
Valve have really knocked it out of the park with the Steamdeck. The console is great and they've made it so my purchasing workflow goes "Can I get it on Steam? Does it work on the Deck?" If the answer to any of those two questions is "no" it doesn't get bought. Simple as. I've even doubled-bought a few games I had free on Epic so I don't need to faff with Heroic Launcher (Control for example). One key point about that by the way is that games through Steam run better because of the pre-compiled shaders being cached. Epic stuff doesn't do that.