To be clear, you can't order one in the UK. Stock has already been depleted there. Not many remain for the states but the link above does work in the US.

For those who don't understand the value of device, think of it like a Roku... a low power, small footprint streaming device but instead of streaming videos from a remote service like Netflix, you're streaming compressed video of your games from your main gaming rig to other rooms of your home. And the link relays controller input back to your rig.

This works best with either wired ethernet or a 5ghz wifi connection. 5ghz wifi is pretty low range however due to its smaller wavelength so getting it somewhat close to a router is important. With a decent setup, I've seen as low as 4.5ms of latency which is practically none at all. Though the link does top out at 1080p/60fps so no 4k on the link. Mileage will vary of course based on how powerful your main rig is, your home network... If your TV has a game mode (to reduce input latency).

As stated above, it does indeed always launch Big Pictures Mode and as for why that would be a problem... It's not. BPM is intended to be an easily navigable launcher much like Playstation 3/4 XMB (XrossMediaBar) to launch your games. You don't need to minimize it to launch gog games or any other game... So long as you add any non-steam games to the steam launcher. If you use steam to launch the title, it will stream (with a few exceptions).

As for controller remapping, the Steam Link relays controller input from any wired and most wireless controllers that are connected to it back to your pc, again with amazingly low latency depending on your network speed. It will simply pass inputs through for games which support controllers natively.. For games with no native gamepad support, BPM now supports controller mapping much like Xpadder or Joy2Key. Before launching the game in BPM you can manage its controller configuration. You can select from ranked community mappings for actual steam games but sadly not for non-steam games you import into the launcher since you can literally just name a game anything you want... Perhaps in a later update, steam will match configurations based on game name and not steam app ID.

But all of this assumes a certain use case.. It's not for people whose PC is within reasonable HDMI cable distance from their PC.

EDIT: Fixed some spelling and adjusted a couple descriptions to be more clear.

It shows up as still available and purchasable for me but I suspect Phin77 is right and there's still stock for the US, which would make sense as I can't actually see Steam UK.

That would certainly make sense. :)

The Steam Link page says "The supply of physical Steam Link hardware devices is sold out" for me (accessing the page from The Netherlands). Though I don't miss out on much in my case, as I'm more fond of gaming in front of my desk with my gaming PC than on a sofa-chair in front of the TV (when I play Playstation games, I need to pull a chair up close to my TV, as a 24" TV is quite tiny when viewed from the couch 4-5 meters width of my living room away from the TV).

Reading "out of stock" for both options (by itself and with the controller) here in the US. No doubt they'll be popping up on eBay by the time Christmas rolls around with people looking to flip a $12 piece of hardware for ~$100+.

I spent $20 on that thing on sale, I now feel like I should have waited.

It works nice though, does exactly what it is supposed to.