I've had mine for 2 days, here's an unedited Adderall-fueled (TMI: I have ADHD and a prescription) ramble I typed to my friends this afternoon.
tl;dr - it's great, just be ready for technical issues, which can mostly just be googled and you'll find a video or reddit post covering it.
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My first impressions on Steam Deck... it's neat, but for technical users only right now IMO. I've had to re-verify integrity of various bits of the OS, had games not start, yolo'd right into trying to install a game and had to delete that because I couldn't install apps in desktop mode because of obscure linux errors (5mb flatpacks requiring 500mb of free space, only 490mb free or something). Once past that... thing is f-ing neat. Have some games installed, and Xbox Cloud Streaming working on it so far.
There's a lot of linux-y things that people respond with to fix problems, that's absolutely not for everyone. I know nothing about kernels, etc, so people saying to use Proton-PE build and whatnot means nothing to me, obviously need to watch more videos - a nice boon for content creators and they've stepped up to the challenge for everything I've needed thus far, including getting emulators setup. I just need to get some ROMs over and probably install a BIOS or two that things like the PS2 emulator needs and I should be golden there. Less of a priority day 1, but something I absolutely want to get setup.
Long term... need a bigger microSD, and may swap out the internal storage once I'm more comfortable. Picking a few games that I'm going to play or revisit largely because of the novelty of the device - like Witcher 3 or Fallen Jedi, though some of those I really want to play on my desktop for the visuals, so will revisit. Have installed several games from Steam that are suitable for the device - Sonic, Rogue Legacy 2, and a couple RPGs (a Final Fantasy and Octopath Traveler). Did that this afternoon and then spent all my gaming time working through trying to get them working in offline mode and overcoming cloud sync errors - required restarts and verifying integrity of OS files... things your nontechnical gamer is going to need a lot of help doing IMO.
So not for everyone, but this is a nice device. It will get there, hope they revise the specs every few years. My primary justification for ordering it is for it to be a more multi-purpose Vita-style handheld. I want to be able to use all the streaming services (and most work currently) as well as a remote play device. Everything else is gravy, but intend to explore the limits.