Okay, now that NDA is lifted i can write my own experience with the device.
Full disclosure:
We (our studio) were gifted with around 30+ Steam Decks and docks with top specs by Valve. I also have my personal Switch, GPD W3 and few others which i use often on the go. Having said that lets get into it.
First impressions:
The device seems bigger then it is. Like when i first got it (like 3 months ago) my first thought was - why it's so big? It isn't, that's just an illusion. Sure it's bigger then Switch but not the point where it's uncomfortable to use or transport. Overal quality is fine to good. Plastic is fine, if you ever used an elite controler this is pretty much the same. Two people at the studio dropped their units while using it and outside of some chipping and a small crack at the bottom on the second unit they're working just fine.
SteamOS is fine, thou i never was a big fan of it.... for obvious reasons the game library is pretty limited if you stick with it. Proton works fine for the most part but it has a lot of problems (performance, bugs, compatibility etc...). So the best option is to just install Windows on Deck and go from there... which i did like a week after i got it. There were no problems with drivers or Windows, everything installed fine and worked as u expect from a... PC.
With Windows i got access to my whole Steam library which is great and the best part was it - most of the game works fine to great on the device. Yes you have to do some detail reduction in very heavy games but for the most part you will get 60 fps without a problem. Worth nothing that the "mouse touch pads" not always work as they should (like a normal mouse input). The screen is bright enough to play in the sun althou if you do it for a longer period of time the device get pretty hot (not to bad but you will feel it for sure).
Dock:
We had pre-production units but they worked great in my opinion better then the Switch one. You can plug in pretty much anything you want to it, mouse, keyboard, ethernet, pads or any other usb or bluetooth device, works the same like with any PC. You can work on it without a problem with a monitor / mouse / keyboard if you want to.
Battery life:
It's better than i expected actually but it varies a lot. If you go with resource heavy game you can deplet it in 2 - 2,5h of playing time. That was the case with Mortal Kombat 11 PC version i tested. However if you play something like Portal 2, you can easily go past 4 hours on one charge and if you play games which are fine with 30 fps you can hit like 8 hours.
Online:
In the handheld mode you can only use WiFi, there is no mobile support with sim slot which is good and bad at the same time but for most people this will be fine. You can use ethernet with the dock which i recomend for any competitive playing... for obvious reasons.
For who it is:
This is the tough question and i think there is no clear answer to it. It's better then Switch in many ways but the same time does lack exclusive titles that you can play only on this hardware like Nintendo has which is a big factor for people considering handheld devices. On the flip side you can play almost ALL PC and Xbox games on it, you can work on it, you can do pretty much anything you want with it, the same like with any PC. Most people at our studio think it is still the best as handheld gaming device and should be viewed as one. It's smaller and lighter than any laptop so you can carry it anywhere you want, the battery life in gaming is also far supperior to any laptop (thou thats mostly coz of weaker hardware). Wiil it beat Switch? Probably not, the reason for that again - it's Nintendo games which people love, like Zelda, Mario etc.... However it will make handheld market much more competitive and open up more options to play on the go.
Feel free to ask any questions about the device.