Steam Machine owner here! As of like three weeks ago.
The Steam Machine's biggest problem in general: it's launching into a headwind, because of hardware shortages. It should have been like $650-$700 for the base model, instead it's $1100ish.
The Steam Machine's biggest strength, overall: the Steam library. Absolutely massive.
The Steam Machine's nerd-gamer appeal: you can also run emulators, use it as a desktop, and play games from other stores (Epic, GOG, et c.). Nb this all comes with a big asterisk, as you're very much into non-console-like operation with any of this—I had to intervene with Heroic's installation of The Outer Worlds (from Epic) once to get it running with functioning cutscenes, which took me probably 30 minutes including a lot of trawling Reddit and forums to figure out the most-recent version of Proton that would work [I doubt I found the most-recent one, but I did find one more-recent than the first suggestion I saw], and then again on about my fourth play session when an update caused it to misplace its own saves (my only indication was all the saves had disappeared—they hadn't moved, it just wasn't finding them there anymore) for some reason, which I doubt would have happened on for-real Windows, and also any time it has an update available it silently fails to launch until it's updated, but I'm not notified of this until I re-start Heroic itself. This is some real computer-gaming and Linuxy bullshit (which is to say, it sucks and wastes your time) not a console experience. Still a benefit that it's there, but comes with a caveat that someone who just wants it as a console won't/can't/shouldn't engage with any of this.
Where the Steam Machine struggles as a console: Yes, it has tons of games available from many, many consoles of multiple generations, running natively (not emulated console games—though, mostly through proton, but Wine Is Not [an] Emulator), including very recent games, giving it a library that dwarfs that of any (... non-"hacked") console. HOWEVER:
1) Couch multiplayer sucks. Very few PC-first games include it, and a lot of console ports will have had it removed (since, who local-multiplayers on a PC now that the Internet is widespread? That hasn't been a normal feature for PC games since, like, the DOS era). If you want lots of couch multiplayer options you're gonna need to deal with console emulation (and I can tell you, bluetooth pairing is better on here than most Linux systems I've thrown together, but it's still bluetooth pairing under linux... hope you like either cords, or headaches) most likely, not just playing Steam games, at which point you're outside the space of treating the Steam Machine like a normal console and into wrestling with a computer.
2) Parental controls basically don't exist. You can have restricted-library child accounts after you jump through some very annoying hoops ("sorry, this just-created kid's account seems to be in another country from the family you're adding it to, based on its purchase activity... which doesn't exist, there has been no purchase activity")... but there's no PIN-locking, anyone can just use anyone else's account unless you want to log all the hell the way out every time you use it. It's a total joke compared to a Switch or PS5, neither of which are, themselves, exactly amazing at this. In short, its account management is tacked-on crap, they do not adequately-well support having multiple people using one of these, even if you don't factor in the various ways a person could get around it on account of the thing actually being a fairly ordinary Linux OS.
Where the Steam Machine struggles as a gaming PC: Hope you don't love playing the very-latest online multiplayer games. Odds are pretty good you'll be excluded by anti-cheat.
This leaves it a damn nice device for, almost exclusively: 1) people like me, a (to put it mildly) power-user who can benefit from its more computer-like aspects while also appreciating just putting it in its default Steam-only environment and relaxing, and who doesn't give a damn about online multiplayer, AND who can afford it; and 2) people who want to treat it as a console and play almost exclusively local single-player games (and have some money burning a hole in their pocket)... and this'll probably only be a great use-case if they already have a largish steam library, since otherwise their needs are likely covered by a Playstation or X-Box and starting from scratch on a steam machine with no owned games won't be very appealing. Which means it's kinda only a great deal for people in this category who were also already PC gamers.
That's a pretty small market of ideal customers, IMO. I'm rooting for the thing overall but I'd hesitate to recommend it to most gamers.
