I've already given my opinion in the chat, but basically I'm not convinced there's much of a market for it and I'm wary of Valve's manufacturing process and software optimization.
The key question is who's the target demographic? And like you said Griff, PC players are already playing on their PCs. So it's the niche that wants to do that on the go, isn't satisfied with their smartphone or their Switch, has the money to spare and is willing to take a bet on a device that is guaranteed to have growing pains. Because it's a Valve product it's sure to have its aficionados (it basically already does before it's even out), and good for them. It will probably be a superior alternative to all the other Switch clones out there, which would be good, although I'm not convinced Valve's objective aligns perfectly with those of that crowd (mostly playing emulated/pirated games). Either way, I don't expect Valve to sell millions and millions of them.
On a personal level, I don't have much need for such a product. I play PC games on my desktop PC. The last three Steam games I've played were Sekiro, Prodeus and Resident Evil VIII. I'm not convinced they'd run on the Steam Deck, and even if they did, it's not really how I'd want to play them. When I think of "PC-first" games, stuff like Doom Eternal, Flight Simulator or Civilization comes to mind. I'd rather not play those on a Steam Deck. I bought a Switch day one for Breath of the Wild, and I remember starting it in handheld mode, playing in bed. After 10 minutes I plugged it to the TV because it felt insane to play a game with such grand scope on a tiny screen. I do play the Switch in handheld mode a lot (probably 80% of the time), but it's mostly indie games or "smaller" games that are adapted to the device, and there isn't one I can think of right now that would be on Steam and not on the Switch.
There's two other things that I think may result in some people being eventually disappointed by the device. The first is that, much like making modular smartphones was always bound to fail (because what defines smartphones is hyper integration of components in as tight and miniaturized a package as possible), you can't possibly make a "handheld PC" that's cheap, small, powerful and has hours of battery life to spare. That's because making things small and portable necessitates compromises on the electronics side.
It's an equation you just can't cheat on. The more processing power you have, the bigger the battery needs to be and even then, it won't do miracles. That's why the battery life is given as "between two and eight hours". In short, anyone expecting to play AAA games at 60 fps for hours on end is probably going to be disappointed. Also, having more raw processing power is one thing, but optimizing for what you have goes a long, long way. Will Valve produce top notch software without any bugs? Steam itself is a bit of a mess as far as optimization goes, and Steam VR was a nightmare for years before they cleaned it up a little. That's also a concern I would have, if I were interested in the product.
More generally, there's currently a huge shortage in the global electronics supply chain, which is affecting everything from laptops to cars. Valve is going to face those same problems, which may be compounded if they have any hardware problems at all. They've gained experience in manufacturing hardware with their VR efforts, but the Index has had a non-negligible return rate, and making a headset with a screen and some sensors isn't quite the same thing as having a SOC, a battery and so on (that means thermal management among other things). On the plus side, AMD's doing a great job these days so maybe they'll knock it out of the park here too. We'll see.
The last element of concern to me is game compatibility. The OS of choice to play video games is Windows, but the Steam Deck will run on Valve's custom Linux. They're working on adding compatibility with more games, but it's not going to be perfect, nor are all those games going to run as well as on Windows. This really should be taken into account. And I've seen people say "oh well, you can also install Windows or other stores on it", but they missed the footnote that says "with garbage performances". I'm sure a great modding community will sprout and cool things will be done with the device over time, but that's not really for the mainstream consumer.
